Number of Fish per gallon/gallons of water?

Question

I was wondering if there is a general rule of thumb with regards to how many fish per gallon or gallons of water?

Answer 1

The general rule of thumb is 1 inch of fish per gallon of water, but it really doesn't work in all situations. If you are keeping small cumminity freshwater fish, that guidline works ok.

If you are keeping bigger meatier fish, they produce more waste inch for inch and have to be calculated differently. Also, saltwater fish can't be kept in that concentration.

Answer 2

It really depends on how big of a fish you're getting. If you're getting a school/shoal or tetras like Neons, you can probably stick about 6-12 in a 10gallon tank with adequate filtration and still be able to put 3 corys in there. However, a 10gal tank is only 24" wide. If you get a fish that grows 6", that fish really doesn't have a whole lot of room to swim and exercise. It'd be like you living in a small room your entire life. There's also something called a bioload. Fish poop and food decays. All of this change the chemistry of the water that your fish are living in. It can actually become poisonous to fish, but luckily there's beneficial bacteria that live in the gravel and everything in your tank that eat these toxins and remove them from the water. Those bacteria are what you're actually growing when you "Cycle" your tank. You still have to change your water, but the bacteria help. If you put too many fish in your tank, you're going to produce way more toxins than those bacteria can handle and your tank conditions are going to get very toxic very quickly. You could, of course, try to overcome this by adding additional filters, but you can only filter the water so much and your fish can only take so much turbulence in the water. Some fish are far cleaner than others. For example, Tetras are not messy fish, but Goldfish are just gross. For tetras, the "rule of thumb" is 1" of adult fish per 1gallon of tank. For Goldfish the rule is 1 fish per 20 gallons. Of course this doesn't take into account the fact that Goldfish get large and should really be kept in 4' fishtanks. This is another reason why beginners should start at a 20gal for a minimum tank size. They're easier to maintain and the fish selection is much better. The honest to gods truth with fishtanks is the right size for you is the largest tank you can afford. I have a 90gallon fishtank downstairs and now I don't think it's big enough. I remember I started with a 5 gallon when I was little. That was a fiasco.

Answer 3

Common Glass Aquarium Sizes

5.5 Gallon 16x8x10 (128 sq. in.)
10 Leader 20x10x12 (200 sq. in.)
10 Hexagon 14x12x18 (127 sq. in)
15 Gallon 24x12x12 (288 sq. in.)
20 High 24x12x16 (288 sq. in.)
20 Long 30x12x12 (360 sq. in.)
25 Gallon 24x12x20 (288 sq. in.)
29 Gallon 30x12x18 (360 sq. in.)
30 Gallon 36x12x16 (432 sq. in.)
30 Breeder 36x18x12 (648 sq. in.)
33 Long 48x13x12 (624 sq. in.)
37 Gallon 30x12x22 (360 sq. in.)
38 Gallon 36x12x20 (432 sq. in.)
40 Long 48x13x16 (624 sq. in.)
40 Breeder 36x18x16 (648 sq. in.)
45 Gallon 36x12x24 (432 sq. in.)
50 Gallon 36x18x18 (648 sq. in.)
55 Gallon 48x13x20 (624 sq. in.)
65 Gallon 36x18x24 (648 sq. in.)
Petite body fish
1” per gallon or 1” per 12 sq. in. of surface
For example 20 long has 360 sq. in. surface area
360/12=30
Can a 20 long support 30” of fish? Probably not.
A 20 high has only 288 sq. in. surface area
288/12=24
Can a 20 high support 24” of fish? Probably not.
A 25 is also 288 sq. in. surface area
288/12=24
Can a 25 support 24” of fish? Probably.
I like to average the two for a more accurate capacity.
For the 20 long, 30 by surface area, 20 by volume averages to 25”
Can a 20 long support 25” of fish? Probably.
For the 20 high, 24 by surface area, 20 by volume averages to 22”
Can a 20 high support 22” of fish? Probably.
Why does the 20 long support 3” more fish that the 20 high?
A larger surface area allows a greater oxygen exchange, which is as important as volume of water.
Large bodied fish, such as gold fish you need to double or even triple these numbers, i.e. 3 gallons per inch and 36 sq. in. of surface per 1” of fish.
This is not the only consideration for fish tanks. Types of fish will appreciate one type of tank over another. For example, fast swimmers like Zebra Danios, Pearl Danios and Blue Danios will do best in a tank that is at least 30” long. They would be better kept in a 20 long rather than a 25. Giant Danios would do best in a tank at least 48” long. They would be better kept in a 55 rather than a 65.
On the other hand, slow, tall bodied fish like Angel Fish and Discus will do best in a deep tank. They would be better kept in a 65 rather than a 55. They, like other cichlids also need a bit more than the 1” per gallon and/or 12 sq. in. surface.

Answer 4

Many people say 1 gallon per inch of fish. This works well for small fish but fails badly for larger fish.

Some people say 5g per inch of fish for bigger fish or 1g per cubic inch of fish.

Thats a good guide however not always good either. For example an Oscar can get 13" long, it needs a 75g tank by itself to be comfortable.

The best way is to research the fish and see what the required tank size for it is.

Answer 5

As general rule of thumb with smaller community fish, 1 inch of fish per gallon of water, not including the fin of the tail. With bigger fish it becomes trickier. These kind of calculations also do not take into account aeration and filtration system differences.

Answer 6

Many people say 1 inch of fish per 1 gallon. However, many topo contributers, such as magicman, do not agree with this. All of those reasons he listed play a role in stocking a tank. Also, to find out a rough estimate as to how many inches you could stock, multiply the length times the depth (front to back) then divide by 12. That's about the # of inches of fish you could have in your tank.

~ZTM

Answer 7

The general "rule" is 1 inch of fish per gallon...

But, that is rarely accurate.


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Answer 8

No there is not. There is a widely circulated and highly inaccurate set of "rules" concerning this, however they so rarely apply to any tank or any set of fish that they are far, far more dangerous to use than to not. Please never base any tank on any of these bogus junk rules. I could write a short book on reason not to use it, but here's just a few examples.

The size and shape of the fish changes the rule
The size and shape of the tank changes the rule
The territorial nature of the fish changes the rule.
The fishes activity level changes the rule.
The level of filtration changes the rule.
The likelihood that the fish will breed in the tank changes the rule.
The fishes ability to interact with certain other fish changes the rule.
The temperature of the tank changes the rule.
The fishes eating habits change the rule.

Starting to see a pattern? The rule simply doesn't apply except in very rare cases and is wrong far more often than right. Please take the time to learn about the needs of the fish you would like to keep instead of trying to rely on a very broken, very wrong "rule"

MM

Answer 9

For fresh water fish it's 1 inch of fish per gallon(size of fish when it is full grown). Saltwater fish need much more room than this! Hope this helps!

Answer 10

It is 1 fish per gallon of water- freshwater
It is 1 fish per 2 gallons of water- Saltwater