Fish size vs. Tank size

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vaguely present
Apr 29, 2003
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I guess more specific info on what is thought to be excreted into the water would be really useful. At least an article or book where I can go look. I personally haven't heard of anything that fish excrete that affects growth specifically, but I am also certain that I don't know everything. I tried searching a couple of biological databases and came up with nothing. Can anybody help me out?


No freelancing!
Aug 14, 2001
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Unfortunately, I don't have the book right now, I loaned it to my sister. Let me see if I can locate the title...Aha!

Fish Behavior in the Aquarium and in the Wild
by Stephan Reebs

It discusses the use of hormones by dominant fish and the effects it can have on tankmates.

This link discusses a study that was done with koi...Can't comment on how reliable it may or may not be, and don't have the rime right now to dig deeper.



so i says to the guy....
Jun 11, 2003
winchester VA
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heres a little formula for finding out how many fish can be in your tank.

take the surface area of your tank( length times width), then divide that number by twelve for tropical freshwater, and that is the max inches of fish you can safely have in your tank.

The reason that the inche of fish to gallon formula doesnt work is because surface area determines the amount of oxygen that dissolves in the water.

so for example. My ten gallon tank has a length of 20 in, and a width of 10. So multiply 20 and 10 and you get 200, divide that by twelve and you get 16.999999999 rounded out thats about seventeen inches.

all freshwater fish need about 12 sq in of space
all coldwater need27 sq in
and all marine need 47 sq in

sertain fish though, like anabantids(gouramis and siamese fighter fish) can live in low oxygen conditions. The reason for this is that they have an auxillary breathing organ called a labyrinth. thats why siamese fighters can live in small cups and such.


vaguely present
Apr 29, 2003
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Problem is that metabolically active tissue scales with volume, not length of the fish. Maybe it should be sq. in. surface vs kilo of fish. Your tank may have room for 17 1" fish (sounds like a lot to me), but good luck getting those 2 8.5" fish in there. Furthermore, the relationship of tank surface to dissolved oxygen will be strongly influenced by surface turnover/agitation.
I remember reading that relationship in my old books decades ago, and it still influences my stocking to some extent, but life is never that simple.


AC Members
Aug 5, 2003
I did partially overcome this problem by using my spreadsheet. Take the surface area, then divide it by 6. That is the worst-case fish-inches that the tank can handle. (This is twice the recomended load by fishdude).

Then I have a separate table of all fishes I know. I derive a "species factor" which is different for each species. As an example, platies will get 2 while we can give 1 (or even less) for cardinals. Plecos get at least 3 if not 4. Take the actual length of the fish when they are fully grown, then multiply it by this factor. Total up these numbers for all the fishes you have in your tank to see if you are reaching your limit.

For my stocking plan, using this method, I ended up with the capacity of roughly 60% when they are fully mature. I have checked with more experienced people in the forums to see if my stock will be ok and so far, it is positive. (Obviously, I didn't talk about this method I'm using).

I've also noted all threads that talks about a tank being overstocked. I used the same method to see what the ratio is, and almost always, it is above 80% - in some cases, over 100%!.

Nice practical example:
- 10 inch Oscar in 10g tank:
- surface area / 6 = 20 * 10 / 6 = 33.3 worth of "fish inches".
- Oscar gets species factor of at least 4, since it is nortoriously dirty. So at 10 inch, it's 40.
- At 120% capacity, this tank obviously will not work.

- 10 neon tetras in 10g tank:
- fish inches = 33.3
- Neons get species factor at 0.8 since it's real thin and clean. Assuming it grows to 1.5 inches, you get 1.5 * 10 * 0.8 = 12.
- At 36% capacity, this tank should be easy to take care of (at least from bioload perspective).

- glittergirl's tank:
- fish inches = 40 (estimate based on his volumn)
- 11 fishes at 2 inches each. (Are these fat or thin fishes?)
- Assuming average, the species factor should be at 1.5. This gives in total, 1.5 * 11 * 2 = 33 fish inches worth in the tank.
- At 83% capacity, this is somewhat crowded. It would require more careful maintenance.

This obviously doesn't take into account how often the water get changed, plant factors nor territorial behaviours, but at least it seems to take care of the dirtiness of fishes quite nicely regardless of their actual size...

I can post the table I use, if anyone's interested...

There... I made yet another attempt to quantify thing that aren't quantifiable... (hey it goes with your job, I can't help it..) Any comments? :D


Jan 28, 2003
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I dont think its really something that can be quanitified against the fishes size. reading alot about the large fish from and the smaller fish here i think the best way to do this is to take the maximum known size of the fish and look at your tank.

will this fish be able to turn round in my tank?

will he be able to move around? this one is a little hard for me to explain...basically is he only going to be able to go round in tiny circles...or if he moves forward an inch is he already at the opposite end of the tank. I would say can the fish move maybe 2 or 3 times its body length before hitting the opposite wall?

and that would be a bare minimum for me for any fish i will keep.


Give it to us raw and wriggling!
Oct 12, 2003
Ventura, CA 93003
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I have to agree with what Cloud-9 said about 5 postings up from mine. I am going through a bit of a debate right now regarding size of tank and wether or not I should keep a particular fish in it or not. My example is a 40 gallon tank and discus. Now I live in an apartment, and I'm pushing it because *grin* we're only supposed to have a 20 gallon in the place. I've always wanted to try my hand at discus, and I think within the next few months I am going to, but a lot of people say they don't recommend discus in anything smaller than 55 gallons. That I definately couldn't get away with where I live. I have also heard that one discus per 10 gallons as a general rule is acceptable. I was planning on going much lower than that, say, two discus, and a very small school of neon tetras or something like that. I also want to find my discus young if possible, at most no more than a third of their potential size. This way they have time and room to grow in the "small" tank I currently tell my landlord I own, and once I find a different place in a year or so I can upgrade the tank and give my fish more room. In my experience keeping fish you can get away with keeping more fish in a smaller tank depending on species, size, and your tank setup. Probably the best advice I can give is know what the potential maximum size of your fish is going to be so you can plan accordingly, have the best filtration you can have (overkill with the filtration if you're going to have a lot of fish), and know the habits and disposition of the fish you're buying. I certainly would not crowd oscars in a tank *chuckle* but small tetras love to school and swim in larger groups.

Oops... Didn't see that second page of postings. What Cloud-9 said on the first page of postings (about twelve or so postings back).
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AC Members
Sep 24, 2003
Kaukauna, WI
I'm skipping a few posts here, so excuse me if I repeat anything that's been said, but I feel the need to pipe up a bit:

Fish growth (like most living creatures) is determined moreso by diet quality and water quality than the size of the aquarium. The 30" arowana in a 29g (30" long!) tank is a perfect example of this. I got my previous oscar at around 15" from a guy who had it in a 38g tank its entire life. So, it definitely can be done.

The catch is that a larger tank with the same maintenance schedule necessarily has better water quality than a smaller tank does, since it holds more volume. A 15" oscar will take longer to muck up a 20g tank than a 75g tank, all else being equal. This is one reason why smaller tanks inhibit growth, though it is indirectly.

Also, feeding quality and frequency will determine growth. A well-fed fish has plenty of nutrients to allow for growth, whereas a poorly fed fish takes stored reserves from its body to survive, while not truly growing.

The inch per gallon rule is very poor indeed. The example of a 10" oscar in a 10g tank is a good showing of this. A better rule of thumb would be the volume of fish (and its waste produced) compared to tank volume. But, weighing all our fish and determining their waste output isn't any more practical than trying to provide a fish as much room as it has in its natural habitat.

So, what I would suggest as a new rule of thumb is this, which I think everyone could nearly agree with (of course, all rules of thumb are inherently flawed in certain situations):

To provide adequate space for your fish, it must have enough clean water in order to allow it to grow to its maximum potential size (within a small margin of error due to sex, genetics, etc.).

If your fish doesn NOT grow to full size, irregardless of tank volume, it has NOT been provided with enough clean water and proper diet.


Gold Dragon
Oct 29, 2003
brooklyn, NY
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I think that is fairly uncommon for larger fish to grow up healthy in a relatively small tank. That arowana account is garbage- large fish need large tanks, that should not be difficult to understand. I completely sympathize with wanting to keep a lot of fish but not having the space or means, but it is really not fair to punish your pets over it by crowding them. I mean, there are plenty of smaller fish that don't require large spaces, so why risk harming a bigger species? And yeah somebody already mentioned this but those formulas can be dangerous if they are followed blindly. What do you guys think is behind this desire to keep large fish in small tanks anyway? Is it just that people think they are more fun to have and so are unwilling to get smaller types? I made the mistake a couple of years ago of trying to keep three oscars in a 40 gallon, and I had somehow convinced myself that this would be sufficient- anyway one died and the other two were obviously discontented, so after that experience I am a bit skeptical of all this.


AC Members
Nov 19, 2003
i would much rather keep a bunch of small fish in a large tank than just a few large fish.

like at first i was thinking of a pictus cat for my 55 althought i cannot get accurate information walmrt says they grow to 6 inches and some other website said 10 inches and other websites had said all in between

im not gonna stuff a 10 inch fish in a 55 gallon tank

so i decided to go with 4 to 6 , 2and1/2 to 3 inch corys (full grown)
sure they arent as pretty as the pictus but they will be happier with friends

also im still looking at loaches
clowns get too big

so i like to keep lots of small happy fish