Fish size vs. Tank size

[vaheelsfan]

I have a question about all of these mimimum size requirements that people always talk about. Don't fish usually grow to the size that their environment allows them? I understand that you can't keep a 6" fish in a 10 gallon aquarium. BUT, just because a fish can reach a size of 12" doesn't mean it has to to be healthy. In the wild, fish grow to the size their environment allows them to. This is why in some places a fish of 12" would be a large specimen, but in another place, the same species would have to be 30" to be considered large. Like I said, I know that there is a point where the environment is just too small, but I also don't see where a fish would have to be allowed to reach it's maximum size for it to be healthy and or happy. Is my line of thought way off base here, or does it make sense or is it somewhere in between?

[mogurnda]

I wouldn't say off base, but fish don't simply look around at their tanks and regulate their sizes accordingly. By and large, fish size is governed by its genetic background. It's true that many fish won't acheive their full size because we don't give them the perfect environment in our tanks, but most will, and I think should be given the opportunity.
As far as regional variation, you may be able to give counterexamples, but differences in size between locations tends to involve selection acting on size variants and causing smaller or larger forms to be more common in a given region. Further, environmental factors such as overharvesting can also skew size ratios in an area.
That being said, you can certainly stress a fish into being stunted, but there may be side effects such as susceptibility to disease.
To give a concrete fishkkeping example of how fish aren't limited by tank size, a customer brought a 30" arowana into the LFS I worked in. Amazingly, it had grown up in a 29, and had just enough room to turn around. It was very happy to be moved to the 200 gallon tank.
That was a lot of words, but the upshot for me is that I try to select species that will be happy at their adult sizes in my tanks.

[Slappy*McFish]

30" arowana in a 29g? geez..thats seems impossible...not to mention very irresponsible of the owner...absolutely rediculous.

The point isn't whether or not it is possible to keep a fish in a tank that is too small for it's potential maximum size....ofcourse it's possible...the point is not "can you", but "should you" keep such a fish in that sort of environment...IMO, no you shouldn't.

[Cearbhaill]

Those "closet children" they find locked up by psycho parents don't grow very well either.

You're talking about the difference between thrive and survive.
It's the difference between a one room jail cell and an estate... you could survive quite nicely in a jail cell all your life eating bread and water, but what sort of life would it have been? You would never have developed physically, you would be socially deprived, miserably depressed- basically just an alive human.

Just being able to keep a fish/animal alive and eating is not what keeping captive animals is all about. At least some thought should be given to providing an environment where they can enjoy their life. This means providing habitat where they feel secure, food suited to their digestion, and room to move and feel comfortable.
Their environment needs to be judged by fishy standards- not human standards. Some species need room to graze, some are solitary, some need a wider variety of foods, some even need predators to trigger certain behaviors. There is a whole, whole lot that we don't know about fishy mentality and what they need to thrive- live the optimum live possible.

I believe it is an awesome responsibility to bring a captive animal (and that goes triple for wild caught animals) into our home. We control every single parameter in their world.
We chose to take them on and care for them- the very least we can do is the best job possible.
That doesn't mean everything can always be perfect- it won't.
But skimping on providing something as basic as room to live is not the way to go. If you're willing to compromise on that- what's next? Substandard food?
You'd end up with stunted, colorless, miserable fish.
Is that why you got into the hobby?

[glittergirl]

Can I just ask - How do you know how much room the fish need? There is a lot of conflicting advice out there.
My tank holds about 13 gallons and the information I got from lots of different sources, all said that my tank would comfortably hold 12 fish of about 5cm. I actually have 11 fish, all about 5cm, or they are expected to grow to about this size. I thought I was doing quite well with the amount of room allowed for the fish, but when I came on this forum, lots of people said my tank was overstocked. What is the best way to tell how many fish will live comfortably in any particular tank?
I have quite a lot of plants in the tank as well and the fish swim through them and around them. All my fish are active and curious, their colours are very rich and defined, and they all eat well. I am only a newbie, but I think they are happy and thriving. :)

[mogurnda]

I am hesitant to give a measure of inches of fish/gal or per surface area, because that gets so contentious. I tend to go for about 1 inch/gallon. Regardless of the way you measure it, you can get away with more smaller fish, because they have higher length/volume ratio than bigger fish, which means there's less metabolically active tissue per unit length. An 8" oscar needs more O2 and makes more ammonia than eight 1" tetras.
The factors I consider limiting are
1. Water quality. This is the most commonly cited reason. More fishmeat in the tank makes more waste. Most tanks nitrify well, getting rid of the most toxic NH3 and NO2. However, nitrate will stress some fish, and messes up your tank's buffering system, making it more acidic. Oxygen is less of a problem in a well-filtered tank with lots of turnover, but see "safety factor," below.
2. Behavior/aesthetics. You want fish in numbers and combinations that will make *them* happy. In many cases this means fewer fish than the tank will hold. Very dependent on the species.
3. Safety factor. If you are pushing the envelope, then what will happen when a snail gets in the impeller of your filter, power goes out, or whatnot? I had a 100 gal, filled with Haplochromis that I had raised from babies, which was truly spectacular until I was away and the filter failed. Heartbreak. You can't expect things to live indefinitely without filtration and heat, but keeping stocking low is good insurance.
By the way, what kind of fish are they? It sounds like you have heard plenty of opinions about your stocking already, but I am just curious about what you have. If they are perky and colorful, that's great.

ed.
After thinking about it, boy, that sounds like an awful lot of fish. I'd keep a close eye on them.

[Mr.Jingles]

fact of the matter: fish need room, give it to them.

inches per gallon is rediculous. think more along the lines of: if I found this fish in the wild, how much territory would it be occupying? Then adjust your tank size to its preffered territory and your budget.

[mogurnda]

I keep editing this post, because it's hard to come up with a response that summarizes what I want to say, and is also useful to a newbie.
if I found this fish in the wild, how much territory would it be occupying I regret having given a numerical value, and wish I hadn't. But where does one start? If you are an experienced fishkeeper, your measure will work to some extent. But if you have not kept fish before, how will you imagine what a fish will need in the wild? In fact, most experienced aquarists do not have access to real data regarding home ranges of different species, and the overwhelming majority have fish in much smaller volumes than they would occupy in the wild.
In the end, if someone were to ask me how to stock a tank, the answer would very much depend on the species and how much room it needs to color up, act "naturally" and, if possible, mate. This is based on years of experience. I would always push for fewer, rather than more, fish in a tank.

[Mr.Jingles]

yeah...youre right. It does take prior knowledge to understand what a wild territoy would be like.

Lets put it this way:

If you want your fish to be happy, put it in a tank that provides alot of room for swimming, enough surface area for gas exchange, and enough room for it to avoid aggression from other fish or enough room for it to not kill the fish with its aggression.

in terms of multiple fish:

if you want all your fish to be happy, make sure the fish dont conflict, their total waste doesn't increase the ammonia or nitrites, and (depending on your taste) they aren't always swimming in mixed groups.

for intance, I consider my 50 gallon crowded with
9 cherry barbs
7 1 inch acaras
4 half dollar angels
3 adult cories
3 adult otos
5 adult shrimp
many 1/4 inch baby platies.

my tank is planted, so space is taken up by plants, but the fish are always swimming together, which makes it overcrowded to me, even tho all the other reqs. are met. the mixed group thing is a personal preference tho.

as the fish get bigger, you should weigh the mixing into your equation more heavily.

[cindywindy]

This reminds me of the "one gallon per inch" rule that we try so hard to negate, and never seems to go away.

If this is so, can one expect to put a 10-inch Oscar in a 10g tank ? Never !

The answer to this and so many questions about providing the most optimal environment for your fish lies in research. Read all you can to find out the size range of the fish, and then if you still have doubts about the size of tank necessary to house such a fish, come here and ask. That is what we are here for !

[Mr.Jingles]

good answer, good answer.

[OrionGirl]

Trying to use native habitat size is ridiculous. For example, many fish are migratory, and may cover thousands of miles. Or, live in a lake, and visit different parts of the lake during different seasons and life stages. That doesn't mean that their needs can not be met within a tank, just that this is how they live in the wild. The opposite is true as well--bettas CAN survive in little tiny depressions that have barely 1/2 gallon of swimming area, but that doesn't mean 1/2 gallon of water in a jar is at all comparable. Rice paddies filter that water regularly, the jar can't.

Length is a tricky one, since it has several implications. the most common, though faulty, is that length directly correlates to waste production. No way! Heavy bodied fish may produce much more waste than a single long skinny fish (compare a 5 inch clown loach to a 5 inch weather loach--totally different body mass, totally different waste production.) Larger mass fish will produce more waste, no matter the length. Second component has to do with living space. Ideally, the fish should be able to comfortable turn around without banging into the sides of the tank. However, this does not mean that super flexible fish can be crammed into tiny tanks, nor does it mean that rope fish and the like require a tank 36 inches in all directions. Common sense should tell you if the fish is cramped in it's quarters.

Of course, activity levels of the fish matters a great deal. Frantic danios are not suited for tanks with little swimming room--they tend to 'carpet-surf' if kept in small hex tanks, for example. An adult pleco tends to be sedentary, so swimming room isn't as big a deal, though the pleco won't like sharing his turf with conspecifics.

Numbers matter as well--as with clown loaches, where one fish is often hidden and sedate, but a group is a rough-housing pack.

There is no magic formula, and the technical literature won't often help. Searching for people who have kept the same fish for at least half of it's natural life span, with it displaying normal behavior and size, is the best advice, IMO.

[punch]

You must take into account the adult size of the fish, as well as how fast it will grow. Get a common pleco at 2" and it will be happy in a 20 gallon tank but a few months down the line it will be cramped. When stocking my tank I tried to take into account
the adult size of my fish, not the juvanile size. Except with my clown loaches as they are slow growers, But I also relize I will have to get rid of them at some point (I'm not looking forward to that time) as my tank will not be of a good size and I can't afford a bigger one.

[Mr.Jingles]

maybe this can become a sticky? eh? put it in the archives or something? I think that maybe someone should look over these responses and validate them or back them up and add some input before it gets archived, if it does get archived.

[yashinfan]

Well I think you're right in a way. My goldfish are probably 2" and 4" long, and my b/f has one that is a good 8" or more (it's HUGE!) Now, my fish when they are in the pond grow to the 4" size from the 2" when they were bought from the store. Obviously, if they were kept there they would not grow that large, and obviously my goldfish'll never be 10" because they live in a pond where they would be picked off if they were that large for half the year and an 8 gallon tank for the other half of the year. They aren't unhappy and don't get sick very often at all. So to me, I think that they do adapt their size to their environment.

[Cloud-9]

It's really hard to come up with a definitive answer as to how much tank you need for a particular fish. The easy answer, but certainly not the practical answer, would be to house the fish in the biggest tank that you can comfortably afford. In reality, we all have to find a balance. We have to balance the needs of the fish, our desire to keep that particular fish, and also the size of our budget.

A skilled and experienced aquarists can get by with a tank that is smaller than what most recommendations would say. Your frequency and consistency with regards to tank maintenance is also a very important consideration.

I do not like to see/read tank size recommendations from very uptight people who would always recommend a tank size that is certainly a very, very, very generous estimate. No, you do not need a 29 gallon tank just to keep one Firemouth! At the same time, I do not like to see/read recommendations from people who want to get by with as little as possible. I believe the answer is to find a reasonable balance. Do some research on the fish and take the following into consideration: budget and maintenance habits. In the end, find a balance and make a reasonable judgment.

[OrionGirl]

As requested above, I'm making this a sticky. It will stay a a sticky until it goes 30-45 days without an additional post, at which time I'll kick it over to Archives and add the link.

One more comment...Keep in mind that the above should notbe taken as the end-all guide to fish/tank selection. Instead, take this advice as a starting point for research and thought into what kind of fish you want, and what environment you want to provide them.

[thom336]

Originally posted by yashinfan
Well I think you're right in a way. My goldfish are probably 2" and 4" long, and my b/f has one that is a good 8" or more (it's HUGE!) Now, my fish when they are in the pond grow to the 4" size from the 2" when they were bought from the store. Obviously, if they were kept there they would not grow that large, and obviously my goldfish'll never be 10" because they live in a pond where they would be picked off if they were that large for half the year and an 8 gallon tank for the other half of the year. They aren't unhappy and don't get sick very often at all. So to me, I think that they do adapt their size to their environment.

There is some research that shows that goldfish may be able to their size to the size of their enviroment. They apparently have these sensors which allow them to see how much space is aroud them. But this is of course controversal evidence, and Im not sure whether I agree with it or not - sounds a bit like something out of star trek to me. However, can I just add that even if it is true then it is still no excuse to keep goldfish in bowls.

And onto what oriongirl said about bettas. I would like to quote Julian Sprung on this, from a piece he recently wrote in Practical Fishkeeping magazine concerning small tanks:

"The increased popularity of the siamese fighter, Betta splendens, has promoted the development of various small containers. This may seem like cruel housing, but the aquarium has sufficient substrate and some plants. Bettas are accostomed to cramped quarters. After all, they naturally occur in shallow puddles and are quite able to tolerate fluctuations in temperature and water quality.
Putting some gravel or sand in the bowl offers substrate for biological filtration and a plant offers additional water purification. No aeration is needed since Betta breathe air by gulping it at the water surface."

But is this an excuse? Surely since we have the ability to provide the bettas with a higher standard of living we should? That goes for the goldfish as well - just because they can adapt to a smaller space, does it mean we should keep them in such small quarters? I think this takes us back to what was written by Cearbhaill about whether our fish thrive of just survive. I think as fishkeepers we have a duty to provide the highest possible standard of living for our fish, to ensure that they thrive, and maybe that means going one up on nature.

[demon_surfer]

i have read that alot of fish excret a chemcial into the water when they pee that limits growth, which i think is where the idea that they will stay the size of their tank came from. however if you do anything like a regular water change these chemicals will never build up enough to actually inhibit growth. im not 100% on this though so hopefully someone with more info can post...i believe i read abit about it on wetmans site but again..not sure lol

[OrionGirl]

Hormones will limit growth to a certain degree, and they are excreted all the time, so they certainly will build up to the point of inhibiting growth. Hormones break down fairly quickly (don't know an exact timeline, but within a few days is reasonable), but they still work on the basis of being constantly refreshed.

However--this is not to say that this is good for the fish. The same method would work to stunt humans, but that wouldn't be very ethical, would it? So why would it be ethical to deliberately maintain a fish in an environment that restricts normal development?

[mogurnda]

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?

[OrionGirl]

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.

http://www.sfbakc.org/koienews/hormones.html

;)

[fishdude]

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.

[mogurnda]

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.

[yhbae]

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

[demon_surfer]

I dont think its really something that can be quanitified against the fishes size. reading alot about the large fish from predatoryfish.net 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.

[as40]

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).

[AquaticEnt]

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.

[dwf73]

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.

[thecareys]

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

[Anne L.]

I'mjust cycling my 10 gal right now, and I was thinking of 3-4 corys, and 3-4 tetras, platys or some such. The tank has plastic plantings and some sculptures.

Would these fish get along, and would this be the right amount for the size of tank? Also, how should I stock the tank, 2 tetras, then 2 corys, and so on. Or is there a better method?

[SwedishFish]

good job at answering the question guys