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Aquarium Stocking And Water Changes


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  • Regular Member

This is an article I wrote recently. It is mainly for freshwater aquariums in general, but definitely applies to goldfish. I hope you guys find this helpful. Let me know about any questions or comments you may get after reading this.

There seems to be a lot of bad information and bad ways of thinking about stocking an aquarium.

Most common mistake and assumption: 'X many gallons can handle Y number of fish'.

There are MANY things that effect the maximum bioload (amount of fish plus food) that an aquarium can handle. The two most important are volume and water change schedule. The volume basically sets a general range for the maximum amount of fish. The most important is water change schedule.

Really the water change schedule in general dictates water quality, which is really the most important aspect. Without water changes Tons of filtration on a 300 gallon tank can't handle even a school of tetras. Massive water changes on a smaller tank with adequate filtration can allow for a larger bioload than the average water change schedule on the same size tank.

There are mistakes being made on both sides when stocking is being discussed. The person simply asking 'How many fish can go in my 29' is assuming that stocking level is based just on volume, and they are not taking into account the water change schedule and other factors. More at fault are all of those who simply start answering the question without asking 'what is the water change schedule?' or even 'What filtration do you have?'. The person asking may simply not be advanced enough to realize they need to be aware of these other issues, but they are asking for help. The others are neglecting to fully address the issue at hand and allowing others to keep looking at stocking in the wrong way.

Filtration does not just clean X many gallons the way the packaging suggests. Filters clean up after fish. A 75 with a school of full grown fancy goldfish needs a lot more filtration than if it was heavily planted with a few low-waste producing fish. Part of stocking an aquarium is providing the filtration to deal with those fish. Filtration can limit stocking. More filtration is never a bad thing. Barely meeting the minimal filtration needs of a certain setup is risking problems down the road. When the only filter or one of the filters stops running and you can't replace it for another week, if you only had the minimal to start with you are now faced with an under-filtered/overstocked tank.

In general the best measure of water quality is nitrate concentration. Nitrate is in general the end of the nitrogen cycle in an aquarium. In freshwater there are two main ways to remove nitrate, water changes and plants. With plants there has to be a very small bioload for that tank size and A LOT of plants. This is rarely the case and even then small water changes are needed. So in effect water changes are the only way to remove nitrate from the system. Nitrate itself will build in concentration over time. This can stress the fish, cause stunting, and can lead to illness and death. In addition, there are many other things that can build up over time. Growth inhibiting hormones are given off by many species of fish which can stunt conspecifics and cause the same problems as nitrate. There are also dissolved organic compounds and other things that slowly build up over time, causing stress, illness, and possibly death. Even if extremes such as illness and death are not caused, a general failure to thrive can occur. This means that even though the fish seem fine, they would be doing EVEN better (sometimes much better) if water quality was improved. In general these chemicals will not build up if adequate water changes are done. Although we cannot test for these other harmful chemcials, they do tend to correlate with nitrate concentration which is easily tested for. It is generally recommended to keep nitrate concentration under 20ppm.

So a more appropriate answer to 'How many fish can I put in my X gallon tank?' without asking any questions would be: 'However many you can have with your water change schedule and keep the nitrate concentration under 20ppm'.

This brings us to another important aspect, the upper limits of aquariums and large fish. Even if you can keep the nitrate concentration down (water quality up), there is still an upper limit to the stocking density. I tried this with a 40high. I did massive weekly water changes and the water quality was in ideal range. However, at a certain point it is simply so crowded that the fish are stressed not by water quality, but by the simple presence of so many other fish in the tank. This became apparent at about 80 fish in my situation in the 40high. At about that population mystery deaths started to occur as well as other signs that the fish were not thriving. I cut down the population and a happier, healthier community resulted. I do not suggest this and it is much better to prevent issues than treat them (don't fill your tank until they start dying to figure out the maximum population).

In addition to simply being crowded, we come to the topic of big fish and their minimal tank sizes. Water quality is still most important. But with big fish and schooling fish, you face a minimum in addition to that of maintaining water quality. In general, the tank should be at least as wide as the longest fish is long. So a cichlid that is 15" long should be in a tank at least 15" wide, to allow for an arguably comfortable turnaround. There are a few exceptions to this. Fish that are very long and extremely flexible do not require the turnaround width like bulkier fish do. These would be fish like ropefish, bichirs, and eels. These fish can be 18" long and turn around comfortably in a 12" wide tank. This does not mean you can crowd a bunch into a 55, but that the minimum width rule is not really in effect, to a point. Schooling fish also require larger than expected tanks. For example: giant danios generally only get to about 4" or so, but since they are so active and do like to school (a school is generally considered at least 6 fish) they should be in something like a 30long at minimum, some would even say a 55. Goldfish are another example. Many suggest 20 gallons for one and an additional ten gallons for each additional goldfish. This is a good guide, but since they do school this means that in order to have fancy goldfish you should really have at least a 75 gallon aquarium. For the more active, and larger, long-bodied goldfish it is generally recommended to have at least a 125.

Fish behavior also needs to be taken into account. An active species needs more space than a less active species of the exact same size. So things like danios, although small, need a larger tank. One-because they are so active, and two-because they school. Aggressive species need special attention as well. Although they can turnaround in a 125 and you may maintain the water quality, five oscars in a 125 will end up most likely killing each other, there is simply not enough room for all of them. This is especially true of cichlids when they pair off, they will become extremely aggressive and may lay claim to the entire tank as their territory. These fish were not made to be in aquariums. Many of them have territories in the wild larger than almost any aquarium they are ever kept in. So when they pair off they are not required to obey your feeling that there is enough room for everyone.

Another commonly overlooked aspect is nocturnal fish. They hide away unseen during the day, but can be VERY active at night when the lights are out and the fishkeeper is not watching. As far as the keeper knows it is a medium fish that hides all the time and is very lazy. Only they do not realize that at night that fish is all over the place, possibly stressing out the diurnal fish that are now trying to rest. This can even cause things like mystery disappearances of fish. People see healthy fish and yet they keep disappearing. The culprit may be that otherwise very lazy catfish that at night goes after the sleeping fish the keeper has noticed disappearing.

An increase in tank size does not mean the keeper can now become lazy about water quality. Yes, a larger tank does mean that water quality will stay a little higher, but that does not allow any laziness on the part of the keeper. Since many upgrades are made for a growing fish, the water quality will still need to be maintained and water changes should remain just as often and as large.

Any fishkeeper needs to occasionally check the nitrate concentration to ensure that water quality is actually being maintained. Many have a tendency to get in the habit of a water change schedule but fail to adjust it to the fish growing. In addition, what seemed like a great water change schedule at first, may actually just mean that the nitrate concentration will rise much slower than usual, not actually maintain the ideal level of no more than 20ppm. If the tank is being under-maintained in other ways there could also be a significant increase in nitrate concentration. If debris is trapped in the gravel and not being removed it will eventually break down and be converted into nitrate. Many or probably most keepers tend to not maintain their filters often enough. This can cause the same problem. The debris they collected will eventually break down and cause nitrate problems. To help prevent these issues and catch them early it is important to check nitrate concentration on a regular basis (for example, once a month) even on tanks that are doing amazingly well.

Many keepers feel it should 'not be fixed if it is not broken'. Unfortunately this can mean that problems that are slowly developing but not causing any major problems immediately are overlooked and develop to the point where they cannot be fixed and the result is harm to the fish, either by stunting, severe stress, or death.

Stocking is far from a very over-simplified quick rule of thumb like 'one inch per gallon', 'one inch of fish per inch of tank length', and other 'rules'. It is very complex and everything from fish behavior to your ability to properly maintain water quality needs to be considered.

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  • Regular Member

just about everything you've said IS what is recommended here on koko's.

the only thing different is you're saying giving a "rule of thumb" or the assumption that "x many gallons can handle y number of fish" is incorrect which later on in your paper you say just rephrased.

two most important are volume and water change schedule

1. all mods and helpers recommend a minimum of 10 gals per goldfish, but if possible do not stock to the max.

2. in addition, all mods and helpers highly recommend that everyone keep a water testing kit in order to keep track of accurate water parameters. if one knows their tank parameters, they are able to set a water change schedule as you'd like to call it.

3. all mods and helpers recommend keeping the aquariums adequately filtered (this in addition to keeping up with water changes)

the list goes on... there are many specifics an individual should know PRIOR to keeping any type of pet, however in this day and age... the majority of individuals act on immediate self gratification basis without adequate research OR worrying about care taking or consequences.

i think you will be hard pressed to find any long time, loyal koko member who has kept healthy fish who would merely suggest that it's ok to keep fish to the rule of 10 gal per fancy, without adequate water changing and proper filtration.

in fact, those are among two of the MANY questions asked in the disease and diagnosis question box.

you say that it is the "Most common mistake and assumption: 'X many gallons can handle Y number of fish'."...

however, based on experience, if a newbie comes here to koko's with the simple question how many fish can i keep in a 20 gallon tank, and you bombard them with your article... most likely...

1. the individual will not read the entire thing.

2. they read it and forget what was said and don't take the advice anyways.

3. they will be turned away from the hobby.

if a new member is here and is open to advice before they've already set up an over stocked, under-maintained tank, they are always advised to get a tank that has enough volume... provide adequate filtration... and keep up with water changing because water quality is #1 in keeping healthy fish. more often than not, the individual is usually here because they've already overstocked and under-maintained the tank and they are in need of help.

as for your paper... i went through and highlighted the parts that i think were your key points.

~ Most common mistake and assumption: 'X many gallons can handle Y number of fish'.

~ two most important are volume and water change schedule

~ More at fault are all of those who simply start answering the question without asking 'what is the water change schedule?' or even 'What filtration do you have?'. The person asking may simply not be advanced enough to realize they need to be aware of these other issues, but they are asking for help. The others are neglecting to fully address the issue at hand and allowing others to keep looking at stocking in the wrong way.

~ In general the best measure of water quality is nitrate concentration?. So in effect water changes are the only way to remove nitrate from the system

~ Growth inhibiting hormones are given off by many species of fish which can stunt conspecifics and cause the same problems as nitrate.

~ In general these chemicals will not build up if adequate water changes are done.

~ So a more appropriate answer to 'How many fish can I put in my X gallon tank?' without asking any questions would be: 'However many you can have with your water change schedule and keep the nitrate concentration under 20ppm'.

~ there is still an upper limit to the stocking density. I tried this with a 40high. I did massive weekly water changes and the water quality was in ideal range. However, at a certain point it is simply so crowded that the fish are stressed not by water quality, but by the simple presence of so many other fish in the tank.(don't fill your tank until they start dying to figure out the maximum population).

so basically you go to say that water quality and volume are the main keys to keeping a healthy tank... which requires adequate filtration, keeping an eye on nitrate levels, and basically a tank can hold as many fish as one can put in there and still keep up with the water changes to keep the nitrate below 20ppm

THEN you go one and say "However, at a certain point it is simply so crowded that the fish are stressed not by water quality but by the simple presence of so many other fish in the tank." this can occur even if it's not the quality of fish in the tank, but the size.

so what i'm trying to get at is if...

1. fish will continue to grow as long as water conditions are good, then eventually it'll lead to

2. goldfish specifically will grow very large even in a tank that is overstocked... which would then lead to

3. crowding which will cause stress... and then inevitably

4. disease and possibly death.

so is it wrong to begin the whole thing by just saying don't overstock your tank and keep up with water maintenance, which is what we preach here at koko's anyway? :hmm

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  • Regular Member

This was not written directly for Koko's, but in general based on what I see happen at many forums I go to.

"so what i'm trying to get at is if...

1. fish will continue to grow as long as water conditions are good, then eventually it'll lead to

2. goldfish specifically will grow very large even in a tank that is overstocked... which would then lead to

3. crowding which will cause stress... and then inevitably

4. disease and possibly death."

I don't really see the point of this. Stocking needs to be based on potential full size, not current size. So although they will continue to grow, that should have been taken into account and never become an issue or have upgrades as needed.

I don't think people looking for help enough to find Koko's will be turned off by this type of article. It explains a good way of looking at stocking and why they may have gotten to the point they are at where they need help, because they used these 'rules'. Then they can get into a better way of looking at stocking. Yeah, maybe some will want an oversimplified version of this, but I think most willing to come here and ask are happy to read through more than just the basics. If they wanted the basics they would be reading some cheapo introductory book or just in the goldfish section of another forum.

"so basically you go to say that water quality and volume are the main keys to keeping a healthy tank... which requires adequate filtration, keeping an eye on nitrate levels, and basically a tank can hold as many fish as one can put in there and still keep up with the water changes to keep the nitrate below 20ppm

THEN you go one and say "However, at a certain point it is simply so crowded that the fish are stressed not by water quality but by the simple presence of so many other fish in the tank." this can occur even if it's not the quality of fish in the tank, but the size."

Yeah, that would be a clarification. The most important things are volume and water quality, BUT this does not make it okay to crowd even if you can keep the water quality up. That was the point of that.

I really do not feel this is just a rephrased version of any rule. I have never seen any rule of thumb for stocking even mention water quality, which is really the biggest point of this article. You cannot underestimate how important maintaining water quality is. Even if overfiltered and understocked, without adequate water changes water quality will go down and there are too many fish for that setup.

Yes, I understand that most on here already have these ideas and work with them very well. But I already had this article and thought I would share it. More information and more to read for people trying to always advance is never going to hurt.

What does everyone else think?

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  • Regular Member

What exactly did you find misleading?

I do tend to write some of them long, but I think it is better to provide all relevant information. I personally wish I had found more long, very detailed articles years ago that would have helped me take better care of my fish back then.

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  • Regular Member

Just the all over tone. I beng a fish keeper for many years can gather you mean the same thing I have been doing. Don't over stock and do water changes.

But someone else might see "If you do this you can keep more fish." Sorry not really good at explaining myself. Which is why I don't do write up about fish. lol

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  • Regular Member

ok, then in that case, i agree mostly with what you have written.

i think however, the part where you tried testing the upper limit of stock density on your 40 gal high part would mislead beginners into thinking

it's possible (therefore they might try) to overstock their tank and keep up with water changes.

i'm not saying all would try, but there are those individuals out there who would justify overstocking a tank with the thought that

"i will just keep up with water changes"... then what happens when they get busy and cannot keep up? or go on extended vacation?

i highly doubt most Americans (who tend to be a bit lazy in the first place) would be capable of keeping up an aggressive water change schedule as you like to put it.

also, if as you say most people who come to koko's are really looking for help and to be informed..

we would have many more members who would actually do research before asking a question like "how many fish can i keep in a 29 gal tank"

answers to questions like that, including the importance of water volume and water changing schedules are mentioned ALL over koko's site and this forum.

in my opinion... learning (anything) requires one to be open minded and willing to accept information be it from a book, from an individual's experience, or from personal experience.

not only to accept it but to actually be able to look at all sides of a situation and analyze the good and bad, identify what appropriate actions need to take place, then be able to make a decision for oneself based on the analysis. a person is not really here to learn anything if all they do is come to the forum, ask a question without having even tried to look up information first OR not doing any research after having been given advice to look up more information, not listening or considering to the advice of others. we can learn in many ways like mentioned previously, from personal experience (having fish die because we didn't know what the heck was going on), learning from books and literature, and learning from another's experience.

in my opinion, the smart thing is to do all three.

and most people coming to koko's for help really aren't interested in reading long articles otherwise they would have read the information (which isn't even long compared to your article)

on koko's website or searched for more information regarding fish care. the people who don't mind reading are the ones who read first, then post on the forum about how they just recently learned the appropriate needs of goldfish but are stuck with some situation or another and then ask for advice.

being a teacher, it takes A LOT to get me frustrated, impatient, or ticked off.

nothing bothers me more than:

1. a "student" asking a question and not really looking for an answer...

2. a "student" when given an answer that is not what they were looking for, choose to ignore the answer...

3. a "student" who asks questions without looking for answers on their own in addition to looking for anther's answer...

basically, lazy "students" who aren't really wanting to learn but to be given a simple answer with no work or effort involved.

(those kids fail my class and i would rather be fired than let them pass)

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