One Inch Per Gallon Rule?

Didgeridoo

Hey there! I've heard some conflicting information about the inch per gallon rule. Specifically, that it doesn't refer to length, but actually sq. inches of fish. This is a sticky over at the Monster fish Keepers forum, and I wanted to hear some opinions about it. This was written by the user "Guppy."

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.

What do you all think about that? It seems to make sense, but then again I am very, very new to fishkeeping. I'm a bit confused...
 

sirdarksol

We've had the discussion before, and the cubic inch thing works better, but it still isn't perfect.
Around here, we call it (or try to) the "one inch per gallon guideline," and we generally only apply it to small, commonly-kept community fish. Even going with the cubic inch, there are fish that don't fit it either due to biology or personality (think bettas)

To go with Guppy's example, a 2' long arrowana would be absolutely miserable in a 100 gallon tank. It wouldn't even be able to turn around without bumping into the side of the tank.

To go with another example of Guppy's, I certainly wouldn't want to cram 50 fish of any kind, even the glowlight tetras, into a 10 gallon tank.

As you can see, there is a bit of a failure in Guppy's logic. (sorry, I couldn't resist mimicking that)

The reason we call this concept a guideline is that there is more information involved in the stocking process than the simple idea of size.
 

docjr03

It does make sense, but I am in the same boat as you, being a newbie and all! It would be nice if there were a more scientific guideline out there that gave us info on the volume of ammonia each type of fish produced...
 

sirdarksol

It does make sense, but I am in the same boat as you, being a newbie and all! It would be nice if there were a more scientific guideline out there that gave us info on the volume of ammonia each type of fish produced...

There is, but the problem is that it takes into account several factors that make it difficult to put down as a simple formula. It involves the fishes' diet, their size, the biology of the species (different animals possess differences in digestive efficiency), etc... That's why, in the end, the best way is to research the individual species when you're making a decision on what to add.
 

prairielilly

I have found that there are a million (ok...a lot) different methods for calculating EXACTLY how many fish you can put in such and such a tank - figuring out what to do can be incredibly confusing! The danger with these methods, as sirdarksol said, is that they don't take into account many things more important than the actual size of the fish - like temperament, behaviour, bioload etc. They also don't factor in things like how much decor you have (more decor, less actual water) or maintenance.

All that said, I always feel it's dangerous in any situation to over-science-ize something, as in putting too much importance on some calculation. The best method (I've found) is to START with the 1"/gallon guideline and come up with a list of what you think will work, then ask more experienced fishkeepers what they think of it. As you become more experienced, you'll be able to determine for yourself what will work and what won't.
 

docjr03

I have found that there are a million (ok...a lot) different methods for calculating EXACTLY how many fish you can put in such and such a tank - figuring out what to do can be incredibly confusing! The danger with these methods, as sirdarksol said, is that they don't take into account many things more important than the actual size of the fish - like temperament, behaviour, bioload etc. They also don't factor in things like how much decor you have (more decor, less actual water) or maintenance.

All that said, I always feel it's dangerous in any situation to over-science-ize something, as in putting too much importance on some calculation. The best method (I've found) is to START with the 1"/gallon guideline and come up with a list of what you think will work, then ask more experienced fishkeepers what they think of it. As you become more experienced, you'll be able to determine for yourself what will work and what won't.

Well said...
 

Didgeridoo

Thanks for weighing in, everyone! I thought the tetra thing seemed a bit over the top. Can you imagine 100 tetras in a 20 gallon tank?
 

docjr03

Thanks for weighing in, everyone! I thought the tetra thing seemed a bit over the top. Can you imagine 100 tetras in a 20 gallon tank?

Imagine???? I have 102!
 

pinkfloydpuffer

When stocking... well, I usually have species tanks, so that makes it easy. But, if I were going for a community, I start with only a few fish. Then wait a while, add a few more. I keep an eye on the water quality. As long as the fish have the proper space, just watching the nitrates should tell you when you're fully stocked. With all of my tanks I can get away with water changes every other week (although I usually do it weekly anyways). For example, my 10 gallon tank. If you look at it in my aquarium info in terms of fish it's quite overstocked. But, they all have sufficient room, and I never see nitrates over 10ppm, even if I let it go 2 weeks between water changes.
So, I say, forget the rule, figure it out through experience! ;D
 

Fla_Larry

FLOG THE SCURVY DOG
 

Dozey

When stocking... well, I usually have species tanks, so that makes it easy. But, if I were going for a community, I start with only a few fish. Then wait a while, add a few more. I keep an eye on the water quality. As long as the fish have the proper space, just watching the nitrates should tell you when you're fully stocked. With all of my tanks I can get away with water changes every other week (although I usually do it weekly anyways). For example, my 10 gallon tank. If you look at it in my aquarium info in terms of fish it's quite overstocked. But, they all have sufficient room, and I never see nitrates over 10ppm, even if I let it go 2 weeks between water changes.
So, I say, forget the rule, figure it out through experience! ;D

Although still new myself this is exactly the “science” I am planning to use. I have a 35 gallon tank and will be stocking it with very small fish so I know room to swim and turn around will not be a problem.

Currently I only have 6 neons, 1 clown pleco and an apple snail. My nitrates never get over 5ppm so I know I’m far from my stocking limit.

I will continue to stock a few fish at a time until I find my nitrates reaching 10 to 15ppm in a week. I will be able to control this with water changes.

Of course I am also considering the habits of the fish I will be adding (where they swim in the tank). I wouldn’t use this method and stock my tank with fish that are only middle swimmers. Then over crowding could become a problem.

So my pleco keeps to the bottom of the tank. The neons are right there in the middle of the tank. And my next addition will be 2 honey gouramis that like to hang out near the top of the tank. And as far as the apple snail well he’s just every where.
 

dancerhas

Great points everybody!

Here's how that formula would apply to our 4'' Chinese Algae Eaters:

2x(4x1/2x1/2) = 2 Gallons

Two 4-inch CAE's in a 2 gallon tank??? HA! In the case of long skinny fish, it's safer to go with one inch of length per gallon of water. This would bring us to 8. Much better, but not even close to what two CAE's need due to their bio-load and swimming habits.
 

gremlin

Great points everybody!

Here's how that formula would apply to our 4'' Chinese Algae Eaters:

2x(4x1/2x1/2) = 2 Gallons

Two 4-inch CAE's in a 2 gallon tank??? HA! In the case of long skinny fish, it's safer to go with one inch of length per gallon of water. This would bring us to 8. Much better, but not even close to what two CAE's need due to their bio-load and swimming habits.

No kidding! CAE's are HUGE fish in a small body! I don't think I would keep one in anything less than a 40 gallon. And if there were two, then I would go with a 75 gallon or larger.
 

Iron waffle

I wish people would stop looking into "rules" for stocking fish, none are accurate enough to be worth following them. Really, common sense is the best guideline...
 

Lucy

Where's Amanda? She dislikes the 1" rule too.

Personally, I like to use it as a guideline, not a rule and only for small non bulky fish. To me, it helps to give new fish keepers something to go on and avoid overstocking a smaller tank.
After someone gets a good understanding of the nitrogen cycle, swim space and compatibility then they'll know what their tank can and cannot handle.
 

docjr03

When stocking... well, I usually have species tanks, so that makes it easy. But, if I were going for a community, I start with only a few fish. Then wait a while, add a few more. I keep an eye on the water quality. As long as the fish have the proper space, just watching the nitrates should tell you when you're fully stocked. With all of my tanks I can get away with water changes every other week (although I usually do it weekly anyways). For example, my 10 gallon tank. If you look at it in my aquarium info in terms of fish it's quite overstocked. But, they all have sufficient room, and I never see nitrates over 10ppm, even if I let it go 2 weeks between water changes.
So, I say, forget the rule, figure it out through experience! ;D

Couldn't think of a better way to do it. PFP seems to have it down!

(I will be using this method when I finally get to start stocking my tank thanks to her advice previously!)
 

sirdarksol

Where's Amanda? She dislikes the 1" rule too.

Personally, I like to use it as a guideline, not a rule and only for small non bulky fish. To me, it helps to give new fish keepers something to go on and avoid overstocking a smaller tank.
After someone gets a good understanding of the nitrogen cycle, swim space and compatibility then they'll know what their tank can and cannot handle.

Well said. This is pretty much where I'm coming from when discussing such guidelines.
 

Nick G

The best stocking rule i've found is to combine the 1 inch guideline with common sense. I have a 75gal tank with 24 inchs of fish, the 24 inchs come from 2 Oscars though and I wouldn't put them in a 24gal tank! I also have a 20gal with about 26 inchs of fish which is 'overstocked' according to the rule, but they're all small fish with small bio-loads who live quite happily together.
 

Drew 43920

Which tank ?

Great points everybody!

Here's how that formula would apply to our 4'' Chinese Algae Eaters:

2x(4x1/2x1/2) = 2 Gallons

Two 4-inch CAE's in a 2 gallon tank??? HA! In the case of long skinny fish, it's safer to go with one inch of length per gallon of water. This would bring us to 8. Much better, but not even close to what two CAE's need due to their bio-load and swimming habits.
Which tank do you have them in ?
 

Dane

I'm really glad that this topic has been opened. I find people are very quick to accuse others of overstocking (although they are often right!) without personal knowledge of the setup etc. Too many "1-inch-****'s" out there! ;-) Stocking is a very dynamic thing, dependent on many variables, agree that is definitely a good base or guideline, which must be used synergistically with common sense, observation and experience (not that I have much of that last one myself!).
 

dennis

When stocking... well, I usually have species tanks, so that makes it easy. But, if I were going for a community, I start with only a few fish. Then wait a while, add a few more. I keep an eye on the water quality. As long as the fish have the proper space, just watching the nitrates should tell you when you're fully stocked. With all of my tanks I can get away with water changes every other week (although I usually do it weekly anyways). For example, my 10 gallon tank. If you look at it in my aquarium info in terms of fish it's quite overstocked. But, they all have sufficient room, and I never see nitrates over 10ppm, even if I let it go 2 weeks between water changes.
So, I say, forget the rule, figure it out through experience! ;D


Even though I am a noob, this does makes sense the most.
 

sirdarksol

I'm really glad that this topic has been opened. I find people are very quick to accuse others of overstocking (although they are often right!) without personal knowledge of the setup etc. Too many "1-inch-****'s" out there! ;-) Stocking is a very dynamic thing, dependent on many variables, agree that is definitely a good base or guideline, which must be used synergistically with common sense, observation and experience (not that I have much of that last one myself!).

I will say that it is extremely rare to go over the one inch guideline and not have an overstocked tank. When we say that it doesn't work, we almost always mean in the other direction, i.e. a 10" oscar won't fit in a 10 gallon tank.
 

dancerhas

Which tank do you have them in ?

30 gallon. We do two weekly water changes to keep up with the bio-load. We bought them when we knew nothing about their potential size or proper stocking. They are going to be moved to a 55 gallon sometime...hopefully within the next month.
 

Drew 43920

Awesome fish..........

30 gallon. We do two weekly water changes to keep up with the bio-load. We bought them when we knew nothing about their potential size or proper stocking. They are going to be moved to a 55 gallon sometime...hopefully within the next month.
They are awesome fish...........
 

Fishlover423

Mine is growing up in a community tank fast,
 

Lucy

Let's get Didgeridoo's thread back on topic. It's not about CAE's.
It's a discussion about One Inch Per Gallon Rule.

Thank you
 

pinkfloydpuffer

Well at least it wasn't me this time, Lucy!
;D
 

Mike19

at the end of the day filtration plays a large role in stocking. good filtration results in good waste management. even with the largest of tanks if your filter is not up to speed it won't matter what you stock in it.
 

Didgeridoo

I'm mainly interested in the rule as applied to neon tetras (or similarly sized schooling fish). It strikes me as off that a little tetra needs an entire 2 gallons to itself, but I'm not sure. That's initially why I thought this version of the rule seemed to make a bit more sense than the usual "1 gallon per inch (lengthwise) of fish" guideline. What is everyone's take on the 1 gallon rule as described in the original post in regards to tetras?
 

Dozey

This is only how I understand it but that doesn’t mean I understand it correctly.

I think the 1” rule is for little more then a planning stage. The reality is that you can only introduce a few fish at a time anyways. So your weekly water readings are going to be a far more accurate measure of your stocking capacity.

Assuming ammonia and nitrites don’t climb above zero…
If your nitrates are hitting 20ppm between weekly water changes you could consider your tank stocked. Or you could slightly over stock and increase the amount of water changed or the frequency of your water changes.

If however you are showing ammonia or nitrite reading between weekly water changes this would be an indication that you have over stocked the tank too far. As long as the reading are still fairly low after a week (0.25 for example) you could likely keep the stock and just increase water changes while the stock thins out.
 

≈ D ≈

snip ..... i.e. a 10" oscar won't fit in a 10 gallon tank.

Sure it will! But it would be a tight squeeze and the fish won't last very long j/k

On a serious note (and staying on topic) I no longer use the 1" per whatever rule/guidline. Tried it, botched a tank with it. But I didn't have the wisdom or experience then as I do now.

I stick to the trusted testing of the tank's water parameters, good filtration and common sense.
 

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