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Phosphate - Possible Way To Limit Algae Growth?


denniss

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I've been thinking. Good plant (or algae) growth requires light (for energy) and three macro-nutrients. These are carbon (comes from carbon dioxide), nitrogen (comes from nitrate or ammonia), and phosphorus (from phosphate). The ratio of C/N/P for optimal plant growth is something like 100/20/1. If any one of these nutrients is missing, or deficient (in relation to the others), it will limit plant or algae growth. Therefore, by purposely causing one of them to be limiting, one might be able to control algae growth.

Can't limit carbon in an aquarium. The fishes respire carbon dioxide continually.

Can't limit nitrogen. The food we feed contains over 25% nitrogen by weight, and the fish produce ammonia waste in pretty large quantities. Plus, you can't remove nitrogen from an aquarium without constant water changes.

Phosphate - now here is a possibility While it takes only a very small amount to support algae growth, it is excreted by fish in pretty small quantities. Unlike nitrate, phosphate can be removed or bound up by precipitating it out of solution by various cations, such as iron. There are various products which can be used (like phosphate removal sponges, phosphate reactors, other media that can be added to filters) to remove phosphate.

I'd hate to keep the brown/green algae that tends to grow in my tank under control by keeping the lights off (I have 130 watts of compact fluorescent light on my tank). I like a nice well-lit tank. So - I'm going to give the phosphate control route a try. If I can get the phosphate levels down to zero or near zero, perhaps I can cut down on the algae growth. Will report back after while.

Dennis

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i believe that resucing phosphates will reduce algae.

Personally, i found interesting results from reducing phosphates in the food.

I used to feed a flake food that contains a small percentage of phosphorus/phosphates (cant remember how it's worded on the package). I had a big brown algae problem. it grew on or the ornaments and the walls and floor of the tank.

I tried not turning the lights on much, but it didnt help.

I then bought some new pellet food, and as a result stopped using the flake. i think there is still phosphates in the pellets, but there is a lot less waste from pellets than flakes as they get eaten whole, straight away (rather than floating around the tank and getting 'lost' in ornaments/filters, eventually contributing to the chemical build up i the water)

I have been feeding the pellets instead of the flakes for about 2 weeks now, by this point normally my tank is covered in brown algae, but now there is only a few barely noticable patches. My conclusion: phosphate control will/'should' effect algae control.

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Dennis, based on my own experiences with planted tanks, that ratio you mention for optimal plant growth is actually a ratio for controlling algae (although I prefer 7:1 rather than 20:1). If algae didn't exist, then you could give plants as much of anything they want and they'll just use what they need. But within the confines of an aquarium, excess of anything outside the ratio feeds unsightly algae faster than the plants. For some reason the ratio works. Too much or too little of one or the other and algae grows faster than the plants can compete with it. Keep it between 7-20:1 and the algae stays away (along with using CO2).

So by limiting the phosphate only, you'll actually just be feeding the algae.

For a long time it was popular belief that phosphates alone caused algae, but only recently has the research been done into the actual combination of phosphate with nitrate at a certain ratio being the key factor. Based on the phosphate-alone theory, a lot of manufacturers jumped on the anti-phosphate bandwagon with a lot of products to reduce phosphate like you mention, but they simply don't work. I mean they do work to reduce phosphates (a straight water change is far more efficient though), but they don't work to reduce algae as reducing phosphates alone is rather pointless unless the ratio can be maintained with the nitrate - which is extremely difficult to do with goldfish!

To put it simply, goldfish need phosphorous in their food, so they eat and poop a lot of phosphate. And a lot of nitrates too. The levels of which we have no control over, so it's virtually impossible to maintain them in a 20:1 (or 7:1) ratio for any length of time beyond a few hours.

PS. The brown algae grows where there's not enough light for the green to take over as otherwise they compete with each other (green wins in strong/long light, brown wins in short/low light), so I'd recommend doing the opposite and leaving your lights on for longer. Green looks better than brown, I think! :)

PPS. Seriously, you can have phosphate readings so high they're literally off the test chart, but if the nitrates are equally high in the recommended ratio = no algae. Eg, 10ppm phosphate and 70ppm nitrate = no algae. But 0.1ppm phosphate and 30ppm nitrate = algae explosion!

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A couple of points -

I agree that increasing light will tend to favor green algae over the brown, and green looks a lot better. I also like keeping the lights up. However, you will still have some healthy algae growths.

The theory of nutrient limitation is that for plants to synthesize more plant molecules, they need carbon, nitrogen and phosphorus atoms in the ratios indicated. If, for example, they have 100 atoms of carbon, and 20 (you may be right, perhaps the real number is lower) atoms of nitrogen, but can't get any phosphorus, then they can synthesize more plant material.

As I mentioned, I think phosphorus offers the best potential for management. However, the tricky part is that, practically speaking, if you have more than trace amounts of phosphorus (the number is between 0.03 and 0.3 ppm), phosphorus is no longer limiting and all bets are off.

So, given that phosphorus is in food and needed for fish growth, can an aquarium's phosphorus removal mechanisms keep the actual concentration below 0.03 to 0.3 in the face of constant excretion by the fish? That is the $64,000 question.

The notion that it is purely the ratio of N to P, regardless of concentration is a new one on me, and I can't think of a theoretical basis for why that would be so. Interesting, though.

I need to get a phosphate test kit and see what I actually have in the tank.

Dennis

(Better living through chemistry!)

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Perhaps you're right in that it's the ideal ratio for plant growth, wherein if the plant is functioning at full capacity then it's outcompeting the algae. But if all the nutrients needed are in excess, then the plant will still use whatever ratio it needs as there's an abundance and so it will still be functioning at full capacity. Yet if any nutrient is in over-abundence that throws the ratio out, the algae outcompetes the plants and smothers them in green film. It's a conundrum that they're still trying to work out why.

So having one or the other either in over-abundance or severely limited - the result is the same:

20ppm PO4 to 5ppm NO3 = algae.

0.1ppm PO4 to 10ppm NO3 = algae.

10ppm PO4 to 70ppm NO3 = no algae.

1ppm PO4 to 7ppm NO3 = no algae.

In theory, you actually *need* PO4 to control algae. NO3 is extremely difficult to maintain very low (below 10ppm) if you have large fish, so you need at least 1/7th of whatever the NO3 is in PO4 (ie, at least 1ppm).

So it's really not a matter of just limiting the PO4 to control algae, unless you can also adjust the NO3 to match the ratio. Any amount of PO4 does *not* cause algae as long as the NO3 matches the ratio. So unless you can also control the NO3 to some extent to maintain the ratio (or close enough to it) continuously, then the amount of PO4 makes no difference as 10ppm of PO4 will cause just as much algae as 1ppm if the NO3 isn't x 7.

There's quite a lot of speculation as to why the ratio works (including that it triggers the plants to give off pheromones to suppress algae!) but nobody is really sure yet. Including me, so I'm sorry I can't explain why it is, but a planted tank expert named Tom Barr (barrreport.com) has information about the N:P ratio for algae control and the origins of it's research. He was actually the one who fine-tuned it from 20:1 down to 7:1 through experimentation (at any quantities at all so long as it's 7:1). When I suscribed to his newsletter and learned of it and tried it the first time myself in my own planted tanks I was amazed as the algae literally disappeared within days. And I do mean 'literally disappeared'. It just died off and disappeared. The previous green film on the front glass became crystal clear. And that was with an extreme 5 watts per gallon of light on the tank (so excess light is also not a factor in algae growth if the ratio is maintained). Plants still growing rampant just as before, but no more algae.

Alas, goldfish tanks are another kettle of fish altogether in trying to maintain the ratio though! To answer your question, I was a true believer in the reduce-phosphate theory myself when i first started and have tried a version of every phosphate-reducing product available, the phosphate filter, the chelating liquid and resin filter pads and media. All to no avail as I can state categorically from my experiences that they're all futile at controlling algae in goldies' tanks. After trying them all (and wasting a lot of money!), and being unable to work out a way to maintain the ratio for more than one day, my conclusion as to the best way to control algae in goldfish tanks is old-fashioned elbow grease. ;)

PS. I 'cheat' and control green water algae with a UV unit. ;)

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AW - Interesting stuff. I presume that the 7:1 ratio works to control algae in a planted tank, because the correct ratio allows the plants to outcompete the algae for available nutrients. My tank went through a plant "bloom" this summer and I had few algae issues at that time (I had plant issues as the plants expanded to nearly fill the tank). I have cut way back on the plants and am working on trying to control the algae as I have indicated.

I picked up a test kit and found phosphate off the scale (which maxed out at 2 ppm). I bought some Seachem phosphate/silica removal resin, but that in a filter and have 0.5 ppm phosphate after 24 hours. My tap water is at about 0.1 ppm phosphate. I will play with this for a bit and see how it works. Actually, I'm interested in finding out if I can get the phosphate consistently below 0.3 ppm.

I greatly appreciate your thoughtful reply, and won't struggle with this too much based on your experience with phosphate removal. Particularly since my tap water is coming in with a trace of phosphate. As I'm converting my goldfish tank to barebottom, it is already proving much easier to keep clean. Therefore, if I need to rely on the elbowgrease method of algae control, it will be a bit more convenient.

Thank you again.

Dennis

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Follow-up - about a week into phosphate control. The brown algae has definitely taken a nosedive. The green algae is growing nicely, taking over from the brown. Looks better, and my water sprite looks MUCH better. As I'm using Phosguard, it also removes silicates, which may be more responsible for the brown algae decline than the phosphate.

Dennis

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