The "one inch per gallon" rule

tlindsey

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Aug 6, 2011
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Hey guys....this is my first post! Haven't read all these but figured I'd put my 2 cents. I've been doing tanks prob 4-5 years but got sucked in quick, learned a lot and still have lots more to do. I heard this formula a while ago and threw it away. My recommendation would be to just put as many fish as you think looks nice and then maintain it properly. I raise all big catfish and I'm WAY over that number but all my fish cram under a log all day and the tank looks empty. That being said, they're big messy eaters so I have 2 FX6 filters, a Marineland hanging filter, and I'm doing 1 to 2 30% water changes, all on a single 125 gallon tank twice a week. Yes...overkill...but when I look at my fish in there and how healthy they are I smile. 😊 and if you have a garden hose and a siphon nearby the water change takes 15 mins. Point is...I don't worry as much about the exact biomass in the tank so much as what I need to do to maintain it. Water changes are life, change your filter media, often. Fish tanks aren't meant to sit there for months with no love. If you're trying to figure it out a safe formula for stocking your tank...there is none, just take care of it.
Welcome aboard 😀
 

catfizz

Feeder Fish
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May 4, 2022
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While I am a long way from agreeing with most of this...I don't think 2 weekly water changes of 30% each on a tank crowded with messy eaters is anywhere near adequate, let alone "overkill"...but your last sentence is solid gold. :)

Many aquarists need to spend more time with a hose and bucket, and less time with a calculator. :)
Thanks buddy.....keep in mind I have 1200 gallons worth of filtration on a 125 gallon tank and I change my filter media every 3-4 weeks. I also feed live minnows that I net in the creeks and any frozen fish I net out after a day to cut down on nitrate build-up. I watch the gills on the red-tail like a hawk, he's the first to get stressed, if he's okay everyone else is. I've gotten advice from fish shops and learned the rest watching fish-keepers online. I know a lot of people won't agree with the fish that I keep, but I'm going to have a pond built in a greenhouse to keep these fish when they reach their max size. I have been obsessed with Amazon catfish since I read an article in the In-Fisherman in January of 1989. I do not half-ass this. I am having a 300 acrylic built as the next step before they need the pond. The pond will be large and deep. I spend tens of thousands of dollars to take care of these fish. I would say to anyone not planning on taking these measures do not buy Amazon catfish or goonch. (Goonch need separate tanks btw with tons of water flow and cool temps)
 

duanes

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I believe the 1" per gallon rule was coined when the average fish aquarists kept, were neons and guppies.
And are ridiculous when applied to large fish.
For larger fish, I believe it should be the 1" of large adult fish, to at least 10 gallons of water.
And fish should be measured length plus width plus height. So if a fish is 10" L 3" W, and 3" tall, it needs at least 160 gallons in tank size.
And that thing about so many gallons of filtration, is hypothetical tripe, especially with large fish.
And filtration is only effective if it is cleaned often, so nitrates aren't spewed back into the tank water.
If deleterious substances are in the filter, they are also still in the tank.
And the most dangerous substances in tank water, are not what you can see, but the compounds you can't see.
 

williamsheikspare

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May 9, 2022
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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.
Creating much more sense.
 

williamsheikspare

Feeder Fish
MFK Member
May 9, 2022
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I believe the 1" per gallon rule was coined when the average fish aquarists kept, were neons and guppies.
And are ridiculous when applied to large fish.
For larger fish, I believe it should be the 1" of large adult fish, to at least 10 gallons of water.
And fish should be measured length plus width plus height. So if a fish is 10" L 3" W, and 3" tall, it needs at least 160 gallons in tank size.
And that thing about so many gallons of filtration, is hypothetical tripe, especially with large fish.
And filtration is only effective if it is cleaned often, so nitrates aren't spewed back into the tank water.
If deleterious substances are in the filter, they are also still in the tank.
And the most dangerous substances in tank water, are not what you can see, but the compounds you can't see.
Agreeing with your statement.
 

vosen30

Feeder Fish
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Oct 2, 2022
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As a general rule, the "one inch of fish per gallon" rule is a handy guideline, not a golden standard to follow. This is a good way to think about the fish-to-water volume ratio for your aquarium - but there are exceptions to every rule, of course.
For every gallon of water in your aquarium, aquarists generally interpret this rule to mean that you can stock one inch of fish for every gallon of water in your aquarium. According to this rule, if you take it at face value, a five-gallon tank can hold fish that are five inches in length. The tank has a capacity of ten gallons, and its diameter is ten inches. The list goes on and on. However, I do not believe it is a perfect rule.
 
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