The "one inch per gallon" rule

fishaccordingtosteve.com

Feeder Fish
MFK Member
Jan 31, 2019
10
2
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51
This is something that comes up fairly often and is rather misunderstood therefore I will attempt to clarify the original meaning of the general guideline of "one inch of fish per gallon of water".

This is a suggested guideline for a well maintained and filtered tank.
It does not apply to all fish as some have differing requirements.

Here is the part that is being misunderstood.

The "rule" does NOT refer to the length of the fish!

The "rule" applies to the cubic inches of fish in the tank.

This means that a 5" gourami should be measured in this manner,
length overall (5"),
thickness, (1/2"),
height, (2 1/2"),
so for this fish you multiply the following, 5x 1/2x 2 1/2, this gives you a total of 6 1/4 gallons of water.

For small fish like glo-light tetras you will end up with something like this,
1 1/2"x 1/4"x1/2", this comes to 3/16 of a gallon (about 1/5), and that gives you 5 fish of this size per gallon (quite reasonable)

For larger fish you end up with something like this, my example here will be a silver arowana at 24" long, 24"x 4"x 1", which gives you 100 gallons of water.

As you can see this works fairly well.

You do also have to apply some common sense and allow for such things as potential growth, the fish types' tolerance for crowding, and of course the width and length of the tank (a 24" gar will not work in an 18" wide tank even if the tank holds 100 gallons).

So please people, accept that this is just a generalized guideline to figure potential stocking levels, not a hard and fast rule.
Also remember that just because you don't like it doesn't mean you should slam somebody for using it.
And lastly, please don't flame someone by saying a 10" oscar doesn't fit in a 10" tank.
Of course it doesn't,
but the rule never said it would.
I still wouldn't want to keep a 5 inch gourami in a ten gallon tank.
 
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Jdempsy

Feeder Fish
MFK Member
Feb 15, 2019
6
11
3
49
This is something that comes up fairly often and is rather misunderstood therefore I will attempt to clarify the original meaning of the general guideline of "one inch of fish per gallon of water".

This is a suggested guideline for a well maintained and filtered tank.
It does not apply to all fish as some have differing requirements.

Here is the part that is being misunderstood.

The "rule" does NOT refer to the length of the fish!

The "rule" applies to the cubic inches of fish in the tank.

This means that a 5" gourami should be measured in this manner,
length overall (5"),
thickness, (1/2"),
height, (2 1/2"),
so for this fish you multiply the following, 5x 1/2x 2 1/2, this gives you a total of 6 1/4 gallons of water.

For small fish like glo-light tetras you will end up with something like this,
1 1/2"x 1/4"x1/2", this comes to 3/16 of a gallon (about 1/5), and that gives you 5 fish of this size per gallon (quite reasonable)

For larger fish you end up with something like this, my example here will be a silver arowana at 24" long, 24"x 4"x 1", which gives you 100 gallons of water.

As you can see this works fairly well.

You do also have to apply some common sense and allow for such things as potential growth, the fish types' tolerance for crowding, and of course the width and length of the tank (a 24" gar will not work in an 18" wide tank even if the tank holds 100 gallons).

So please people, accept that this is just a generalized guideline to figure potential stocking levels, not a hard and fast rule.
Also remember that just because you don't like it doesn't mean you should slam somebody for using it.
And lastly, please don't flame someone by saying a 10" oscar doesn't fit in a 10" tank.
Of course it doesn't,
but the rule never said it would.
Great post
 
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TimSteel

Feeder Fish
Jun 25, 2019
2
0
1
54
This is something that comes up fairly often and is rather misunderstood therefore I will attempt to clarify the original meaning of the general guideline of "one inch of fish per gallon of water".

This is a suggested guideline for a well maintained and filtered tank.
It does not apply to all fish as some have differing requirements.

Here is the part that is being misunderstood.

The "rule" does NOT refer to the length of the fish!

The "rule" applies to the cubic inches of fish in the tank.

This means that a 5" gourami should be measured in this manner,
length overall (5"),
thickness, (1/2"),
height, (2 1/2"),
so for this fish you multiply the following, 5x 1/2x 2 1/2, this gives you a total of 6 1/4 gallons of water.

For small fish like glo-light tetras you will end up with something like this,
1 1/2"x 1/4"x1/2", this comes to 3/16 of a gallon (about 1/5), and that gives you 5 fish of this size per gallon (quite reasonable)

For larger fish you end up with something like this, my example here will be a silver arowana at 24" long, 24"x 4"x 1", which gives you 100 gallons of water.

As you can see this works fairly well.

You do also have to apply some common sense and allow for such things as potential growth, the fish types' tolerance for crowding, and of course the width and length of the tank (a 24" gar will not work in an 18" wide tank even if the tank holds 100 gallons).

So please people, accept that this is just a generalized guideline to figure potential stocking levels, not a hard and fast rule.
Also remember that just because you don't like it doesn't mean you should slam somebody for using it.
And lastly, please don't flame someone by saying a 10" oscar doesn't fit in a 10" tank.
Of course it doesn't,
but the rule never said it would.
The tetra guide says you can keep 5 Oscars in a 55 gallon tank the pamphlet inside says that for real
 
The tetra guide says you can keep 5 Oscars in a 55 gallon tank the pamphlet inside says that for real
These pamphlets and books are basically useless. They are written by people that work for the company and want to sell.
 

TimSteel

Feeder Fish
Jun 25, 2019
2
0
1
54
These pamphlets and books are basically useless. They are written by people that work for the company and want to sell.
I know but I could see a new fish keeper buying this tank at Walmart then going out and buying 5 Oscars or 3 that it says you can do in the 29 gallon
 

DiscusFish.com

Feeder Fish
MFK Member
Jul 13, 2015
8
18
8
35
It all depends on the setup, amount of substrate, number of plants, filtration system, etc. There's no hard and fast rule.
 

JacinLarkwell

Feeder Fish
Nov 28, 2019
1
0
1
20
I always knew about height and lentfh, but never considered width for some reason. That makes everything fit better now
 

jjohnwm

Aimara
MFK Member
Mar 29, 2019
750
1,482
134
Manitoba, Canada
Wow...what an amazing thread! I got through the first few pages and then stopped to make sure that I hadn't accidentally logged in to Fishlore by mistake...:)

Okay, here's how it works: You take all your fish and dump them into a clean plastic pail...no water, just fish. For obvious reasons, you must move quickly. Now weigh the bucket'o'fish, and then replace the fish into their tanks. Try to put them back into the same tanks you took them out of. Weigh the now-empty bucket and subtract to get the actual total fish weight.

Now, take that weight (in pounds...sorry, Australia!) and convert it into inches, using the formula P(2#) x (T-
Pi) + 3 x F/A , where P = pounds of live fish, T = temperature in degrees Kelvin, F = the magical Fish Constant, which can be found on the internet and varies from species to species, and A= altitude above sea level. Now take the result (which will be expressed in square feet), and divide by 144 to convert to square inches. (I won't even get into the whole square inches versus inches squared thing...).

Take the resulting number and multiply it by the length of your aquarium in inches, then divide that answer by the width of your aquarium in feet. Now measure the height of the tank in furlongs, take the square root of that and add it to the total you just calculated. If you live in the southern hemisphere, you will need to add a negative sign to this result.

Now, the important part: measure the pH of your source water, and take the inverse square root of that number. Multiply it by the previously calculated total, and then add 4, just for fun. It is often suggested that barometric pressure needs to be factored in, but that would just be silly.

Now, calculate the average speed at which your fish typically swim. I won't go into detail on how this is done, as it should be intuitively obvious. Take this velocity, convert it to rods-per-nanosecond, and then double it. Now square the answer, and then take the cube root. Multiply your previously calculated total by this number and then just convert it to a base 8 number. This will be the number of cubic parsecs of water the fish in your bucket will require for health and happiness.

It should go without saying that this calculation needs to be done at least weekly, to keep up with the growth of the fish. Re-calculation is also advised after each fish death or addition of new stock.

See? It's not nearly as complicated as some of you seem to want to make it...
 
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