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Pond Filtration


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The question comes up with everyone setting up a first pond:  "How do I filter it?"  My purpose here is to describe some of the options for filtering a small (1,000 gallons or less) goldfish pond.  There are many variations on these basic filters, and once you have built one you can quickly understand how to upgrade to a more sophisticated system.


(Edit  6/8/2016) Contents of the Thread


In-pond Filters ............. Post #1

Bucket/Barrel Filters ....Post #3, 9,10, 12

Bog filters.......................Post #13, 16

Trickle filters...................Post #17,18, 19


Let's start primitive with the in-pond filter.  The simplest is the basic box filter, like this one.    A pump fits into the box, surrounded by bioballs.  If you read the product description, you will see that this is suitable for ponds "up to" 700 gallons, LOL.  It is suitable for a water garden with no fish where one just wants a fountain or a waterfall.  I had one of these in my first 100 gallon pond. It's quite adequate for biofiltration in a lightly stocked small pond.
Since the water only comes in the top of the box, debris and mulm from the bottom of the pond never makes it in unless you stir it up and make the pond all murky.  If you have an above-ground pond you can siphon the bottom.  The water coming in the top does protect your fish from being left without water in case of a plumbing problem that tries to empty your pond.  The water level won't  drop below the top of the box.  Unless your pump has a low water level shut off, the pump will be toast but the fish will be fine.   The box I have is still in use, after 12 years, as a prefilter in a container pond.  I have made some prefilters (which are used to collect the crud before it gets to the filter), but none of them work as well as this one.  
The next type is the all-in-one submersible which includes a pump and (basically) sponge filters.   Some, like this one, have relatively modest claims saying they will filter up to 400 gallons (with no fish, of course).  Then you have some that claim to filter up to a 1500 gallon pond with less than a cubic foot of sponge.  

If you keep cleaning these filters so they don't get clogged, they will provide really good mechanical filtration and nice clear water. That's why you get all those rave reviews. That may mean daily, and certainly at least weekly, cleaning. I expect they also provide good bio-filtration in a small properly stocked pond. I don't like cleaning sponges, so these don't appeal to me.
Finally, it's easy to build a submersible filter that will provide adequate biofiltration for a small. modestly stocked pond. There's one I made in post#7 here. You can make such a filter in a bucket or tote, put in a pump with a fountain, surround it with filter media and get very adequate bio-filtration. The challenge comes with mechanical filtration. If the water enters the filter near the top, you don't pick up debris from the bottom. If you make the holes for the water to enter near the bottom, when you pick up the bucket and pull it out to clean it, dirty water pours out back into the pond. One woman reported, on a pond forum,solving this problem by using two buckets one inside the other, drilling the holes through both and spacing them so you could twist one bucket and close the holes. She claimed it worked, but I haven't tried it.

Next, the bucket/barrel filter.

Edited by shakaho
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  • 2 weeks later...
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While a submersed filter can work in a small pond, an external filter is much better and will be necessary as the number and size of the fish increase. Before going into the basic types of external filters, I would like to take a look at an aquarium HOB filter as seen from the point of view of a pond person.


At the very bottom of the uptake tube we see a little strainer. In pond terms, this is a prefilter. A prefilter is anything that provides mechanical filtration in water that is on its way to the pump.

The intake tube basically siphons water into the chamber that holds the pump. The pump pushes the water into the filter chamber where the water flows up through mechanical and bio filter media. This chamber of the HOB filter performs exactly the function of a pond "upflow filter." When the filter chamber is full, the water outflows by gravity.

A HOB filter is not just a filter, it is actually a filter system, an all-in-one unit that includes inlet, prefilter, pump, plumbing, filter, and outlet. In a pond filtration system, these different parts are separable.

Here is a diagram of a pond filter system from Laguna that makes both the filter and the pump illustrated.


Notice first that the filter is at one side (right) of the pond, and the pump is at the opposite (left) of the pond. This set-up produces a flow of water from the filter outlet to the pump. Debris rolls slowly from the filter toward the pump, so the pump is picking up the dirtier water in the pond.

Looking at the pump in this system,


what we see is a "case" that serves as a prefilter. The actual pump is inside. The case excludes solids that are large enough to clog the pump. Water from the pond flows into the case, is taken in by the pump, and spewed out the gray outlet that you see on the right side of the pump. That is where you attach the hose that takes the water to the filter.

Look back at the filter system diagram and you see the hose going to a commercial filter, the "Biosteps 10." This is not an upflow filter as in the HOB, but is a horizontal flow filter. Whichever way the flow goes, the filter has certain parts. The inlet, on the right of the filter diagram goes to a vertical pipe inside the filter box. The water goes down the pipe. The up pipe is open at the top. Suppose it were not. Then you would have a closed tube, from the pond to the filter, that is totally filled with water -- a siphon. If the pump should turn off, that tube would siphon all the crud you have so carefully collected in the filter right back into the pond. With that up pipe open, the filter box will only drain to the level of the inlet/outlet pipes.

Observe the outlet on the left side of the filter box. You should notice two things, both of which are there to avoid overflow. 1) The outlet is a little lower than the inlet, and 2) the outlet pipe is larger in diameter that the inlet pipe.

Between, we have the filter medium. This filter uses Matala, rigid pads of interconnected plastic wire. These pads provide both mechanical and biofiltration, going from inlet to outlet, the pads get progressively denser. The pads can be removed individually and rinsed to remove debris.

Notice at the bottom right of the filter box, you see a drain (or dump) valve. This allows you to flush the dirty water from the filter. You can attach a hose to this and drain the fertile water to your garden.

Here is a picture and description of this filter. I use this filter for illustration because the diagram is very good and allows me to point out the features of a pond filter, not because I am recommending it. It does look like a well-designed filter for people who are determined not to build their own. :)

The diagram shows the filter outflowing into a waterfall, a very popular option. Notice that the waterfall is lined so that all water returns to to pond. If the waterfall isn't watertight, it can drain your pond.

More container filters to follow.

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Every time you post something like this, I copy it into a word doc and store in in my "pond" file for future reference! I am not in a position to have a pond now, but when I return from overseas I am definitely planning to have one. Thanks so much for always sharing your experience and knowledge.

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Thanks for posting all of these. I am planning to build an above ground pond with my brother, and filtration is definitely something I need good info on since I know nothing about it.

How much filtration do you want on a pond?

Thanks :)

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Ponds do tend to get quite "messy" due to the fact they are outside so airborn dust and dirt particles leaves inscts etc etc collect in the pond then the added fact of sunlight which increases algae growth. Good and bad.

I was told (by a 30yr pond specialist) what filter to get but managed to score a much larger than unit for much less so grabbd that. He said it will work but is overkill. Ive since Learned that even the larger unit isnt enough.

So work out what size is recommended by the specialist an atleast double or triple that.

I believe they try selling under sized units so you come back for water chemicals etc.

But if you have a proper size filter etc chemicals aren't needed.

Im about to set up a 50gal barrel filter for my pond soon.

Hope that helps a bit Narny.

Sorry for hijacking the thread.

Edited by blackmoors
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Thanks for posting all of these. I am planning to build an above ground pond with my brother, and filtration is definitely something I need good info on since I know nothing about it.

How much filtration do you want on a pond?

Thanks :)

At least 2x GPH of the size of your pond.

Example: 100 gallon pond you want 200 GPH

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OK. I'll remind people that what you call a filter in an aquarium is typically a complex of a filter and a pump. You can't describe "the amount of filtration" by the gph of the pump. You can with a HOB because the filter component is sized to the pump.

The rule of thumb for a DIY container filter is that the volume of the container should be at least 1/10 the pond volume. With this size filter and moderate stocking (30-50 gallons per goldfish) you should have a pump that claims a turnover rate of 2-4 times the volume of the pond. In relating the pump output, pond volume, and filter size, you have to balance the retention time of the water in the filter (the longer the water stays in the filter the more complete the water purification) with the concern about getting all the water in the pond filtered often enough. In theory, the ideal balance between these is turning the pond volume over every 1/2 to 2 hours. Generally, smaller ponds should have higher turnover as should more heavily stocked ponds. Putting too much pump on your filter can lead to overflows.

Furthermore, the pump output is on average about 1/2 what is claimed. Primarily that is because they measure the output of the pump just sitting in the water -- no tubes, no prefilter, no filter, and no need to lift the water to get it to the filter. Another consideration is that pumps claiming less than 250 gph, in my experience are trash. They often use such small tubing that their output is much less than than 1/2 what they claim, and they don't last long.

Commercial filters are different. I've never had one and never will, so I can't help you with those.

I'm sorry I'm not getting this out faster, but these posts typically require about an hour a paragraph to compose.

Edited by shakaho
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I want to talk about the "Skippy Filter."  This was designed and popularized by a guy who owns a shop called "Skippy's Beautiful Ponds and Gardens".  He has a very extensive web site that is well worth reading.  This includes a very detailed description of how to build a Skippy filter.  
The Skippy filter is a basic form of a "sedimenting upflow biofilter."   But many people refer to any sedimenting upflow biofilter as a "Skippy" or a "Skippy type" filter.  Some people do build a filter exactly like what is described in the Skippy site, and it works just fine.  But others, once they build a filter, start thinking about how to improve it.  
I don't know if the Skippy was the first filter to use a swirler to aid sedimentation, but it certainly popularized this approach.   If you just run a pipe to carry the pond water to the bottom of the filter,  the water will pour out, and fill the tank moving in a disorganized fashion.  Particles suspended in the water will go up with the water until they are stopped by filter media.  Unless you have mechanical filtration in the bottom of the filter the biomedium  collects debris.  
In your HOB filter, you have a sponge at the bottom to provide the mechanical filtration, reducing the debris in the biomedium.  You could do the same thing in an upflow biofilter, with one little problem.  Filter pads will eventually clog, and to clean them, you have to take out the biomedium, then the filter pads,  Clean the pads and reassemble.  It's the same as cleaning your HOB, except it's 100X larger.  
Here's a better picture of a swirler.
The pond water comes down the vertical pipe and goes out the two horizontal pipes.  The elbows direct the water in opposite directions producing a slow "whirlpool."   This produces an organized or "laminar" flow of the water.  The horizontal movement of the particles gives more chance for gravity to settle them.  The larger the pipe and the slower the flow, the better the particles settle.  The swirler allows one to do without filter pads for mechanical filtration.  If one takes apart a working sedimenting filter without disturbing the solids (hard to do) one finds clear water and little debris at the top, and then increasing debris as one goes down through the media.  Only at the bottom is the water really dirty.
A swirler works best in a circular container, and the taller the container, the better the sedimentation.  So one of the improvements that many filter builders make is to use a 30 or 55 gallon plastic drum for the filter container.  I was amused that the author of the Skippy site said that he tried using a 55 gallon drum and it didn't work.  It works for everyone else, LOL.  
The biggest complaint most people have about the Skippy filter is that it doesn't have a dump valve, that is, a valve near the bottom of the filter container to drain out the collected crud.  The designer of this filter says it doesn't need cleaning and should never be cleaned.  He says that the biofiltration improves as the filter matures and cleaning sets it back.  There is some truth to his point.  The brown fuzzy crud that one finds in a filter is really a decomposition factory in which bacteria, protozoa, and little animals are cooperating to break down the waste.  The black stuff in the bottom of the filter,  mulm, is basically aquatic compost.  It's not a source of water pollution, but is great for your garden.  You can remove the mulm by opening a dump valve without destroying the active sites of decomposition.  The few people I've heard from who actually ran a skippy for years without cleaning, then looked inside, have found inches of mulm in the bottom and promptly installed a dump valve.
Recall from post #3 above, the value of breaking the siphon where the water enters the filter.  The Skippy does not do this, so if the power goes off, the water in the filter drains back into the pond.  
You can see a contemporary bucket filter that corrects some of the shortcomings of the Skippy in this thread.  It has a dump valve,  the inflow cannot siphon, a stool keeps the biomedia out of the sedimented waste in the bottom of the filter.
The use of uniseals  in the dump valve and water outlet make this filter so much faster to build than the older ones.  The uniseal is very quick and easy to install and forms a tight seal on a curved surface without needing any kind of sealant.
Now we can look at the filter made by Kayla, for her 300 gallon stock tank pond.  The build starts on page 3, but there's a lot of discussion before, which will be useful for any pond neophyte.  This filter uses a 30 gallon drum.  Here are her materials: http://i202.photobucket.com/albums/aa173/kayla111_02/IMG_5115.jpg

Kayla's filter uses pvc ribbon for her biomedia.  Here's an example of this type of medium.  I love this stuff -- miles of surface area, light weight, rinses clean very easily.


The video in her post #60 shows the filter in action.


The quality of biofiltration can be greatly improved by putting plants in the top of the filter.  Yes, they will use up the nitrate, but phytofiltration (phyto means plant) is much more extensive than just using nitrates and phosphates. Plants differ in their ability to take up pollutants, but are an important part of bioremediation of soils and water.  When you dispose of dead plants or dying leaves, you are removing the materials the plant has taken up from your water, which can include pesticides, other organic toxins, heavy metals, and many other pollutants,.  


The rule of thumb for sizing a filter in a barrel, bucket, flowerpot, or urn is the volume of the filter should be at least 10% of the pond volume.  








Edited by shakaho
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  • 7 months later...
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Filter media for an upflow biofilter.

The ideal medium for an upflow biofilter will be plastic, light weight, and have lots of surface area. While you can toss a bag of ceramic medium from your aquarium into the pond filter to seed it, this is not a suitable medium for a pond filter. Your aquarium filter has a small volume and high-velocity water. The pond filter has a large volume and slow velocity water. Under these conditions, a thick biofilm will form over the outside of the ceramic rings, with little water exchange within all those pores. The ceramic rings will house as many biobugs as a plastic bead of the same size, at hundreds of times the price.

My favorite commercial biomedia is PVC ribbon. This is made by shaving thin layers off a pvc pipe in a continuous ribbon. The stuff I bought was made by a local DIYer. He pointed out that the material was super cheap, but the labor was substantial. This media is so easy to clean. Just scoop it out of the filter in one mass and swish it in a bucket of water.

A very cheap commercial biomedia are sponge cubes. I haven't tried these, but they do not impress me. This looks like something that would get dirty and be very hard to clean. If you want to use this, it is an easy -- if tedious -- DIY project.

Bioballs are very popular, but it takes a lot to fill a filter (1000 balls is about 5 gallons).

There are many things that people use for biomedia:

plastic pallet strapping -- that really tough stuff that they hold heavy boxes together with.

plastic mesh scrubbies

mesh produce bags

nylon window screen, cut in strips and loosely knotted

nylon mesh bath puffs

plastic bottle caps

plastic drinking straws, cut to 1 1/2 to 2 inches.

cut up irrigation tubing

scraps of pvc pipe

Easter basket "grass"

Plastic rope I cut it in pieces about 18" long, fused one end with a flame and unraveled to make masses of fibers.

Most of this material will float and thus flow out of the filter into the pond. Thus you need to do one of two things: put something in the top of the filter, under the outflow pipe that will confine the media beneath it; or put the media in mesh bags. I have used plastic baskets, cut to fit, or Matala filter pads that both keep the filter media in the filter and provide support for plant pots. You can get mesh "lingerie bags" for a dollar or two or you can buy "media bags" for ten times that price.

Another filter media that some people like is lava rock. It's inexpensive and easy to find. The problem with lava rock is that once wet it's heavy and it's very hard to clean. Nevertheless, there are many people who love their lava rock media.

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Bog Filters
A bog filter is a type of upflow biofilter that uses gravel as biomedium and plants growing directly in the gravel to add phytofiltration as well.  The single best source of information on bog filters is here.  


(Edit,  shakaho 4/29/15.   While gravel is cheap and is probably the best choice for a large bog filter, I have found that hydroponic/aquaponic growth medium such as clay pebbles or grow stones, promote much better plant growth.  They are also very light weight, which you will really appreciate after lugging bags of gravel.  For a small bog filter, the extra expense is well worth it.)
While upflow biofilters using plastic media function best in a tall cylindrical container, bog filters work best if they are shallow with a very large surface area.  The total depth of the bog filter should be 12-18 inches, with 6-12 inches of gravel.  The bog filter can be deeper if the gravel is held up by a grid with a reservoir of incoming water below the grid.  If the water below the gravel is more than a few inches deep, you should provide aeration to the reservoir.
Bog filters are most often used with in-ground ponds.  Lets look at the diagrams from Nelson Water Gardens.
The diagram above shows a side view of a in-ground pond with a built-in, or partition, bog filter.  The hole dug for the pond includes a shallower part for the bog filter.  The entire hole is lined.  Notice that the surface area of the bog filter is about 30% of the area of the entire surface of the pond/filter system.  The next thing is the plumbing.  The pump does not have two pipes coming out of it.  The diagram shows two ways of moving the water from the far end of the pond to the bog.  The one I (and Nelson WG) prefer is shown with striations.  A flexible hose takes the water from the pump to the base of the bog filter, where it connects to the pipe that goes into the bog filter.  The piping inside the bog filter has holes in it to release the water into the gravel medium.  The end(s) of the holey pipe(s) are capped so the water goes out the holes.  The pipe goes through a wall made of stacked bricks, blocks, or stones that are not mortared together, so the wall is leaky.  
If I were building this wall, I would only use blocks with vertical holes through them like these bricks


or blocks
 so I could run rebar through them vertically so I know they will stay in place.  I have heard of stacked partition walls collapsing.  The partition wall can be made a little lower than the walls of the pond so that water can also spill over the top of the wall from the filter into the pond.





The top view of the partition bog filter shows a rather poor view of the "holey" pipes or distribution pipes.  The arrangement of these pipes depends on the size and shape of the bog filter.  Here is the distribution pipe for the bog filter in my back pond.




Nelson's recommends cutting slits on the top side of the distribution pipe, which is what I did..  Some people prefer drilled holes to slits, and some prefer to cut the slits/holes in the side or the bottom of the pipe to keep the gravel from clogging the holes.


to be continued...

Edited by shakaho
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Good little read. Personally after faffing about with different types of filter I am planning something like a bog filter with a large volume. I'd like to pump water from a clean area of the pond as I'm sick of cleaning pre- filters out constantly. I think this time I will base my water movement on overflowing into different sections of the pond

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This thread is so informative, i think it should get pinned! :D

Sharon answers so many questions about pond filtration, and it would be nice to have a go-to resource like this somewhere handy!

(That's right, i lurk in the pond section imagining the dream pond oasis I willl build in my backyard one day, and fill with beautiful goldies the size of my head :rockon )

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Container bog filter


You can create a bog filter for a pond or aquarium in a container.  While a container bog filter and a bucket/barrel filter have the same plumbing, they differ in shape.  The ideal sedimenting upflow barrel/bucket filter is a tall cylinder which gives maximum sedimentation.  The ideal bog filter is shallow with a lot of surface area to allow for lots of plants.    A container similar to this makes a very nice bog filter for a small pond.  A sturdy tub like this can handle a larger pond.  A 12 inch tall 50 gallon stock tank can make a bog filter suitable for a 500 gallon pond.


Here are two threads in which I describe constructing container bog filters:






In each case, if I were building these again I would use hydroponic growth medium instead of gravel.


A request: I know several people have built bog filters, but I can't find their threads.  If you are one of those people, would you please provide a link to a thread with a description and/or pictures of your filter.  Thanks!

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Trickle Filters


All of the filters above qualify as upflow filters.  Trickle filters belong to the downflow class.   The water enters a downflow filter from the top, usually distributed through something similar to a spray bar, trickles through media and leaves the filter for the pond at or near the bottom.  Since the medium in a trickle filter has full exposure to the atmosphere, the nitrifiers get all the oxygen they can use and then some.


You can see some of the variety of structures people have built as trickle filters here.  A "classic" design uses

.   You observe that the trickle tower in the video had quilt batting in the top box to provide some mechanical filtration.  They also used a sponge prefilter on the pump which they  cleaned daily.  In the absence of some mechanical filtration before the trickle filter, it will need a lot of cleaning.


I built one of this type which I used after the upflow biofilter. My purpose was not more filtration, but rather to produce a quieter and more pleasant sound.




You can see where the water came out in this picture.








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This trickle filter uses a "rail planter" designed to sit over a deck rail.




I covered a 2x4 with plastic to keep it from rotting, drilled holes in the bottom of the "legs" of the planter, and filled it with expanded clay pebbles.  Then I set the planter on the 2x4 and planted it.  


Behind the planter, you see on the left a biofilter, and my fresh water reservoir ( 10 gallon Rubbermaid Roughneck) on the right.  I put and elbow on the outspout of the biofilter, cut a pipe long enough to reach most of the way across the planter, drilled a bunch of holes in the bottom side of the pipe, put a cap on one end and pushed the other end into the elbow.  All of the nitrate-rich water from the biofilter goes through the planter to the pond.  You can see the water disribution pipe here.




You can use a rail planter the same way on an aquarium.  Just run the outlet hose from your canister filter to a distribution pipe across the planter.  Better yet, :)  build a pond filter and distribute the water from that outflow across the planter.



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I used these rail planters on the back pond in an attempt to correct the nitrate problem there.  This pond sits in deep shade, which keeps the water cooler in the summer.  However it doesn't even grow a good coating of algae on the sides.  The plants that I can grow in the filters must tolerate rather low light.


The pond wall has 1x6 pvc boards holding the liner in place.  The rail planters fit over either a 2x4 or a 2x6 rail, so they will fit.  In this case I only want the water flowing out one "leg" of the planter -- the one that is over the pond. 


Here are three of these planter filters doing their thing.




Oh, yes, what a mess of pipes.  The peach-colored 30 gallon filter on the right receives its water from the peach-colored pipe coming out of the pond.  The pump is at the far right of the pond.  I split the outflow from the filter with a tee-fitting that sends pipes to each of the planters on the left side of the pond.  


The black container to the right of the big filter is one of these.  It is too shallow to make a good grow bed, but it fit in the space.  I plumbed this just like my mini aquaponics system,  which you can see in the background.  Half of the water from the small filter on the right gets piped to the bottom of the tub and comes out of a holey pipe across the bottom of the tub percolates up through the growstones recycled glass medium and goes out a pipe to the pond. The rail planter to the right of the small filter gets the other half of the water.  It holds an assortment of herbs including basil, thyme, and parsley.




A recent picture shows some plant growth.




It also illustrates the importance of maturing the system.  In the back of the black tub you can see a tomato plant.  On the other side of the swimming pool, you see a tomato plant in my mini aquaponics system in its second year.  The plant goes off the top of the picture.  I planted these on the same day.  


Yes, the nitrate level has gone down since I set up these planters.


Want to see some fishies?


Here is Butterfly, Lacey, Spot, Bitsy, and Li'l Buddy.  Butterfly was the smallest fish with the longest fins in the 15 cent tank at Petsmart.  She's about 7" sl, with a tail at least as long as her body.  She must be 6 years old now.  Lacey is one of my babies from 2012.  Probably my longest fish, she runs about 9-10" sl.  Spot, a wakin, is the same age.  Bitsy was the runt of my first batch of fry.  Golden, the largest of batch, was at least twice as long and much heavier when they were small.  She's maybe an inch longer than Bitsy now, but much heavier.  Li'l buddy, a Walmart find has always been on the small side.




I have fancies in the back pond too.  Here you see butterfly again (she likes to get in pictures) and a home-grown fantail, Flower.  The narrow end of the pond that you see behind them is 22" wide.  Shimmer, a sakura fantail, is coming toward us.  This is not one of her good pictures.  Then in the forground, Valentine gets his name from the heart on his head, which doesn't show well here.  I got him from the same tank at the same time as Butterfly.  He was 2 or three times her size at the time. Then Bitsy, and my only fancy who came from a store, Cheers, the pompom from (gasp) Petco.  she's also the last fish I bought, about three years ago.  Then Spot and the front half of Fatty, who may be my most massive fancy.








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  • 5 years later...
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5 hours ago, ErnestinaDimarco said:

Once you know the volume of water in your pond you can start to consider your options. A word of caution here! Please be aware that a US gallon is 3.7854 liters, whereas an Imperial gallon (UK gallon) is 4.54609 liters. This issue may need to be considered when you are looking at product specifications for imported products.

This thread died back in 2016. You're reviving necro threads. Try and look at dates on threads before you reply.

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