Does tank size affect fish size?

Paul B

7 Mar 2011
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Long Island New York USA
Do fish grow to the size of the tank?

There is so much debate on this topic that I was hesitant to start it because of all the arguements and furniture throwing I know it will start. (there are some forums I won't put this on)

The answer to the question is a definate, maybe, well, sometimes, kinda, yes, no, and I don't really know. We can try to research it and read through scientific studies to come to a definate answer. But scientific studies usually don't mean much with something that requires a lot of time because research projects don't last for many years. Especially something for a hobby called "Ornamental Fish Keeping". We really don't need ornamental fish so there isn't mush money going toward research.

I Googled it and it seems that "most" (but not all) of the answers say that tank size does not affect the growth of fish unless it is a goldfish. I guess goldfish have a lock on stunting their own growth to the size of their tank. Well, if goldfish can do it, why can't a moorish Idol or bluefin tuna?

To get the correct answer to this question I can only go by my own experiences as I have kept way more fish than I can count for the last 60 years. A good number of them I kept for 10, 15, or 20+ years. (I also killed more fish than Starkist Tuna)

Most of the literature you will find on this will state that fish are stunted because of either inadaquate water conditions, lack of the proper diet or poor health. I will admit that those things would most likely limit the size a fish would grow. But they are not the only things as I will "try" to explain later.

A fish needs to do three things, grow, spawn and have immunity to most of the pathogens it encounters in the sea. Remember that the water the fish is in is a continuation of it's circulatory system so anything that is in the water, could easily be in the fish. It's immune system is the only thing that keeps it safe.

Fish also have to spawn and do it constantly because a fish is an animal that never dies of old age, they are all eaten alive by something bigger. If a fish gets old and slow, it gets eaten by another fish, seal, bird or polar bear. We eat the rest of them. That is why fish have so many babies, to replace all that are lost.

For a fish to acomplish those 3 things it needs the correct foods and a lot of it. Its immune system alone requires an aweful lot of calories but not as much as reproduction. Fish constantly develop eggs and even if their is no male around, they produce eggs then they re absorb them and grow new ones. This takes an enormous amount of energy. Growth is the last thing a fish needs to do so it can mature enough to lay eggs and pass on it's genes. Out of those three things, growth uses the least amount of energy and calories because fish growth is much slower than those other two things.

Fish eggs can be a large proportion of the weight of the fish and fish can spawn every couple of weeks. The immune system constantly produces slime which is a large part of it's immune system because it's slime is infused with white blood cells and antibodies and antiparisitic chemicals to repel and kill offending organisms.

What does all this have to do with the original question? Good question.

Basically all of the answers you will find about "if fish become stunted by the size of the tank" will say that the reason is lack of proper diet, or less than perfect health.

I am saying that if the fish is spawning, immune from diseases, and fairly old, it is as healthy as it can be in a tank. So that, to me, indicates that if a fish is in a tank for a number of years, it is immune from disease and is spawning, that fish is healthy so if it's growth is stunted, it is not because of lack of health.

So why all the controversy and charts indicating what size tank we should have for certain fish? The answer is actually simple but the only way we can find out the answer is by keeping a variety of fish in a tank and see if they grow to the size they would in the sea.

I have done this with many fish, salt and fresh and I can answer with a fair amount of certainty that "It depends on the fish". That is the answer.

Some fish will continue to grow no matter what we do. For instance I have kept flounders, sea robins, remora's, arowana's, scats, and a variety of catfish over many years numbers of times. Those particular fish will outgrow whatever tank they are in, and will do it very fast. I had a remora that grew so fast I could almost hear it grow. It grew over a foot in a few months.

But, (and there is always a but) I have kept many other fish for decades and they never grew very much at all. I will give examples. A hippo tang I had for 7 or 9 years in a 40 gallon tank. He stayed about 4" long. I transfered him to a 6' long tank where he lived to about 12 years old and never got much over 5". That fish in the sea can get twice that size. I had a percula clown for 12 years and for most of those years he spawned. He grew to about 2". In the sea those guys get huge. I now have a fireclown. He is about 25 years old in my 100 gallon tank. He is about 3 1/2" and his 17 year old mate is about an inch shorter. That pair spawns about once a month. As I said, if a fish is spawning, that means it is healthy and it's water conditions are healthy so if the fish is not growing, something else has to be the cause. My copperband butterfly is a few years old and has been about 5" for the last couple of years. I think he is still growing, but very slowly.

Goldfish exude substances in a tank and even in a pond that limits their size. If you put a one inch goldfish in a fish bowl, in 10 years it will be almost the same size. But if you put that same fish in a lake, it will grow to a foot or more long.

Could it be possible that other fish can do the same thing?

I really don't know, but neither does anyone else. We can read all about tank sizes for particular fish but if you personally have kept that fish in a smaller tank for 20 years and the fish was healthy and spawning and it never got very big, why would you think it needed a larger tank?

Yes, of course fish should always be in a larger tank, preferably the ocean so virtually "all" the fish we keep should be in a bigger tank. I get that.

So Tang police (I know you will be all over this like stink on a two week old dead flounder) if someone wants to put "their" own fish, that "They" bought with "their" own money. It is "their" fish and can do with it what they want. I eat fish almost every day.

Tangs almost always live in a school so, if we put "any" tang in a tank, and if you are of the mindset that fish have feelings, it won't be happy no matter what size tank it is in. From my experience most tangs are the type of fish that can remain healthy and live in a smaller tank that is many times recommended, If that tang is small when we put it in the tank. "Some" types of Tangs, like all other fish should be in a larger tank. But this hobby sometimes, either because of finances or space, will not allow for a larger tank. If you would like to disagree, that is of course fine. But before you do, get a tang and put it in a smaller tank than is recommended, keep it healthy in that tank for 12 years, then disagree and at that time let us know what were the problems you had with that tang in that tank.

Of course you also need to keep the same type of tang in a larger tank and feed it the same thing for those 12 years.

I have done this so I have an opinion on it and it is not from reading it from Google.

Now there are the people that will say it is cruel. Remember before you go there, we are all keeping fish captive against their will. They will never contribute to the gene pool in the sea. They are all in a too small tank eating something they are not used to.

I would love to keep my fish in a 10,000 gallon tank, that is not going to happen. But I would love to do that as I want the best for my fish. All my paired fish are spawning so I know they are healthy and I assume happy. :cool:


Regal Tang
23 Feb 2015
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Great read. Thank you Paul.

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2 Jun 2015
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Interesting read!

I know fresh water fish release hormones that stunt the whole populations growth, they actually shrink if needs be, I have seen it with my own eyes in bass populations in the western cape! As far as marines are concerned I doubt it, the water is constantly diluted so I don't see this could be an effective method of effecting growth.