Tank Size For Various Fish

[smmetz]

I've read that a 10-12 inch fish is OK to put in a 75 gallon tank, in general. It fits in the 4x the length, 1.5x the width rule of thumb.

Why is it that a 4-5 inch fish is never recommended for a 10 gallon tank then?

Space-wise, the 10 gallon situation allows more room. (same length, but 2x the width instead of 1.5x)

I imagine some people will say the 10 gallons of water will get too dirty too fast to keep it up well.

Well, if we want to start talking bio-load, consider this. You have a 5 inch fish, and a 12 inch fish. Both are the same as far as ativity level, body shape, etc. The 12 inch fish is 2.4 times longer, wider, and taller than the 5 inch fish then. This means it has more than 13 times the bio load? (2.4*2.4*2.4) Well, a 75 gallon tank is only 7.5 times more water than a 10 gallon. Therefore, the 75 gallon tank should be worse off.

Now - before I get flamed - I am not advocating putting a 5" fish in a 10 gallon tank. Rather, I am asking: Do people bend the rules for larger fish, just because if it so damned expensive to get really big tanks? Should a 12 inch fish really be in a tank bigger than 75 gallons?

 

[Callum]

a 4-5 inch of fish is never reccomended for a 10 gal because if you keep a larger fish in a small tank its not as nice as keeping several small fish. i would never bend the rules for a large fish except i have a BN in a 20gal but he will be moving to my 50gal. i see your point about the 1.5x and the 2x thing but i would generally never keep a large fish unless i had a large tank for instance i wouldnt keep clown loaches in less than a 1.5m tank (length). Money is a big object for me i have very little as im a paperboy but i can still afford a large tank ive just ordered me a huge rena a very expensive tank . so i reckon it is down to space in house rather than budget.

 

[smmetz]

BN in a 20 gallon is a PERFECT example of this. People will say that a pleco up to 12 inches is OK in a 75 gallon. However, a BN, which only grows to 5 inches, is not OK in a 20 gallon? This seems very contridictory to me. Proportianally, the BN has TONS more room. So, either the BN really is fine in that 20 gallon, or the 12" pleco should be in a larger tank, like a 6 foot x 2 foot tank or something.

 

[SLC Flyfishing]

Do you have a 12 inch in a 75 gal? I only ask because it would be interesting to test and see if the nitrates actually do go up quicker for this scenario than for the 5 inch in the 10 gallon scenario.

SLC

 

[CFC]

The difference is volume of water, a 75g has 7.5 times more volume of water than a 10g but a 12" fish is only 2.5 times larger than a 5" fish, it makes a huge difference in terms of water stability.

 

[SLC Flyfishing]

aha so your saying that the length x width x heigth thing doesn't exactly pan out?

I've never had fish that large so was just curious.

But I suppose that a large part of the bioload depends on the species and their eating habits etc. Take puffers for example, a 3 inch GSP would ruin the water much quicker than a 3 inch cory wouldn't it.

SLC

 

[CFC]

Any of the so called "rules" of fish keeping are just guidelines to set people off in the right direction, none are concrete and all can be bent if you are aware of the consequences and know how to deal with them.

The 1" per gallon rule works out for smaller fish but you obviously cant use this rule for larger fish, a 12" fish is just going to look silly on a 12g tank, so you move up to the 4x1.5x1.5 rule, now that 12" fish doesnt look so silly in a 48x18x18" tank. Really its all about common sense, if a fish looks cramped it probably is.

 

[SLC Flyfishing]

Yes I know all that and I've made myself look silly in pointing out something that is obvious to everyone else here too.

SLC

 

[smmetz]

SLC - The largest fish I own are a couple 5" plecos, so I cannot do any tests like that.

CFC - I am only guessing from my limited knowledge, but a fish that is 2.4 times longer must make much more than 2.4 times the waste. I was proposing that when you have two fish of similar proportions, and one is 2.4 times longer - it is also 2.4 times wider and 2.4 times taller. Therefore, my conclusion was that it would have 2.4*2.4*2.4 = 13x more bio load. As I stated in my first post, this assumes similar attributes of the two fish, as far as activity, diet, etc. This is pure speculation on my part though.

I realize that there can not be any rule of thumb that applies to all situations. However, I still don't understand why a 12 inch pleco in a 75 gallon is OK if a 5 inch BN in a 20 gallon is considered bad. Perhaps my speculation on the difference in bio-load is just not true.

For me - I will try to er on the side of caution. If my common plecos grow too large for their current tanks - I will skip a 75 gallon and go straight for 6 foot tanks. (I know they can get bigger than 12 inches anyway)

edit: I know that CFC has many many tanks, has some quite large fish, and has been fishkeeping for a while. He knows better than me what works and what doesn't - I am just trying to understand what, in my ignorance, appears to be a contradiction.

 

[KeeperOfFishyPeace]

so, how would a 5-6" fish act in a 20 or 25 gal environment?

 

[smmetz]

Well, that depends on the fish, the other fish in the tank, and the dimensions of the tank.

Many would say a 5" fish would be fine in a 20L tank - assuming that the tank is not overstocked with other fish. Some would say a 5" fish is OK in a 20H tank. EDIT: usually people consider a 20H tank too short for a 5 inch fish

The only 25 gallon tanks I have seen are the same as a 20H, but extra tall. So, they don't really allow any more swimming room - just a bit of extra water to help slow any changes to the water chemistry.

 

[CFC]

Personally i would see no problem with keeping a bristlenose plec in a 24x12x15" aquarium (around 20g) provided the tank was light on stocking of other bottom dwellers to prevent territory problems. This fish doesnt need room for free swimming and once it has established a area to defend will rarely move more than a couple of feet away from its home even in the wild.

CFC - I am only guessing from my limited knowledge, but a fish that is 2.4 times longer must make much more than 2.4 times the waste. I was proposing that when you have two fish of similar proportions, and one is 2.4 times longer - it is also 2.4 times wider and 2.4 times taller. Therefore, my conclusion was that it would have 2.4*2.4*2.4 = 13x more bio load. As I stated in my first post, this assumes similar attributes of the two fish, as far as activity, diet, etc. This is pure speculation on my part though.

The ammount of waste a fish produces really depends on what its diet is made up of and how much it eats. My predatory fish for example are only fed twice a week with enough food to make them noticably full each time, a large water change a couple of days after feeding removes and "waste" and keeps the nitrate and dissolved organics in the water at low levels. Plec's and other herbivors and omnivors on the other hand will spend several hours of each day grazing and producing a continuous stream of waste which is constantly adding to the nitrogen cycle.

Yes a 12" plec will create more waste than a 5" plec, but a 12" shovelnose catfish will make less waste (in solid form at least) than a 5" plec.
This is a topic that can be argued around and around and never get anywhere, the variables are so great that a one size fits all answer cannot be achieved. As ive said before, use common sense and you cant go far wrong, if you think your fish need more room then they probably do.

 

[andywg]

The 4x1.5x1.5 rule is about fitting the fish in the tank rather than bioload. Most people with larger tanks with large fish will start overfiltering and more frequent water changes.

 

[nmonks]

This seems perfectly good science. As length increases, volume will increase by a cube. So doubling the length of an animal increases its volume eight-fold. So a 20 cm fish will be using 8 times as much oxygen (and demanding 8 times as much filtration) as a 10 cm fish of similar shape. In other words, even scaling up slightly can have a big difference. A 1.25 cm fish will be using up about 2 times as much oxygen and filtration as a 1 cm fish (because 1.25 x 1.25 x 1.25 = 1.95).

Metabolic rate differences between inactive and active fish are not as significant as you would imagine. Because they are cold-blooded, their metabolic rate is determined more by temperature than activity. Warm-blooded animals (like mice for example) can alternate between a normal state and a low-energy torpor state (like hibernation). Fish don't do this. So a 30 cm plec even though somewhat sluggish is likely to be using up oxygen (and therefore producing waste) at roughly the same amount as a 30 cm cichlid.

The key exceptions here will be air breathers. Things like gouramis, snakeheads, lungfish, bichirs, and gar will be able to tolerate substantially lower oxygen concentrations than non-air breathers like most cichlids, tetras, and barbs. That said, contriving an aquarium where a fish had to depend on atmospheric oxygen just to stay alive would probably be very unwise in the long term.

Cheers,

Neale

CFC - I am only guessing from my limited knowledge, but a fish that is 2.4 times longer must make much more than 2.4 times the waste. I was proposing that when you have two fish of similar proportions, and one is 2.4 times longer - it is also 2.4 times wider and 2.4 times taller. Therefore, my conclusion was that it would have 2.4*2.4*2.4 = 13x more bio load. As I stated in my first post, this assumes similar attributes of the two fish, as far as activity, diet, etc. This is pure speculation on my part though.

 

[smmetz]

The 4x1.5x1.5 rule is about fitting the fish in the tank rather than bioload. Most people with larger tanks with large fish will start overfiltering and more frequent water changes.

I agree. So, let's assume that the tanks are properly filtered, maintained, and stocked - so that the amonia, nitrites, and nitrates are under control.

Most people say a bala shark is OK in a 75 gallon minimum. Our own fish index states this, although it does say that larger is preferable. Let's say a bala grows to 12 inches.

Now consider the red finned shark. http/www.fishforums.net/index.php?showtopic=41902 , which grows up to 6 inches. It recommends a 36" tank, minimum. Can someone keep one in a 20H tank, which is 24" long? In my opinion, the answer is yes, the fish can live in a 20H tank, however it is really much better to have more room to swim.

What I am saying is this. I believe that nobody mentions the physical posiblity of putting a red finned shark in a 2 foot tank, because a 3 foot tank is only slightly more expensive. There is no excuse not to get a 3 ft tank.

However, once you get over 55 or 75 gallons - the price skyrockets for the tank with all accessories. I believe it is because of this price issue that people usually are understanding about keeping larger fish in comparatively more cramped conditions than smaller fish.