The One Inch Per Gallon Rule

[symon_say]

Hi

I was reading in another forum "mosterfishkeepers" and the member "Guppy" post this:

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).

I know little about fish but a lot about measure and this seen to be a better/proper way to aplly this rule, cause it takes into consideration the mass of the fish instead of only the length, measure that don't take into consideration the other dimensions of a fish. I know this isn't to accurate for a lot of fish and that there are a lot of consideration, but this might be the way that rule should be apply.

What do you think?? Want to hear from more experimented people in here.

 

[Rynofasho]

That person has wayyyyy too much time on their hands --- no one can be bothered to measure their fishes length, girth, etc. That rule is just a rule of thumb quoted to the masses as many people do not adequately maintain their tanks.

So long as you have good filtration / circulation and you change the water once per week and keep up with maintenance, you can safely keep more than 1" of fish per gallon of water.

The trick is, be honest with yourself. Everyone likes to think that they will be perfect fish parent forever and ever amen and religiously maintain the tank, so they go out and put boatloads of fish in there. If you honestly think you can keep up with maintenance, throw a few more fish in, but if you know you won't always be able to properly care for it, that rule will keep you safe.

 

[Curiosity101]

TBH I'd only ever use it as a guide, never a rule.

And I only suggest NEW fish keepers use it. And even then only for your bog standard small bodied tropicals.

It doesn't really have much of a place on these boards anyways. Cause we'll talk stocking ideas through with anyone who wants advice. And there is alot more to stocking than just 1" per gallon (even if you do do it by full mass as opposed to just length).

 

[symon_say]

I know all that you're saying and if you're dedicated to your fish tank you'll know who many fish you can put in, given the time you have to spend to keep fish healthy, but i'm talking about the general rule, that we found everywhere, not tha fact that it works or not, i just think little that guy say, that the shouldn't be called "1 inch of fish per gallon" it should be called "1 cubic inch of fish per gallon", no one will measure all the fishes, but can do an approximate about it, i did it and i think it works really nice.

Simple 2 neon tetras that are 1.5" grown up won't put the same load like a dwarf gourami that is suppose to go about 3" grown up, then again the neon tetra is small in all sizes and it'll give a lot more fish that you can hold with this formula, but i think it takes more into consideration than the length rule.

 

[Aqua Tom]

 

[Katty]

I think the "inch per gallon" rule makes a lot more sense that way... Like 12 neon tetras is no where near the same load as a foot long fish.

Anyone know where that rule originates from? Like is there any actual reasoning/science behind it other than an inch of fish-flesh per gallon seems like an ok amount? It kinda just seems like something an LFS came up with to get customers out the door when they're busy without going into a big discussion on it.

Also, as a newer person to this hobby I haven't really gotten stocking quite figured out yet. How do you more experienced folks do it? I know there's more to it than inches, like how active they are or whatever, but is there a method/system to deciding or do you just look at each case holistically?

Edit: I actually tried playing around with the numbers, using my corydoras as an example and seeing how I felt about the numbers... 2.5x.5x.5=.625, so 16 full grown corys in a 10 gallon using the cubic method, and 4 in a 10g with the linear inch method... Neither of those numbers feel quite right to me but I'd much sooner suggest 4 to a beginner (which is who you'd be telling "one inch per gallon" to anyways) rather than 16... Yikes. I think it might work better with the bigger fish but beginners (usually) don't buy giant fish.

 

[Uriel]

I researched each fish individually both here and on other web sites and then asked some questions here. Then went to my LFS and had a look at stock and decided on anything i fancied and noted the name down. Then i researched some more. Then i bothered people here with even more questions. Then after my fishless cycle had finished i finally went and bought fish.

Even sq inch per gallon "rule" isn't going to give you anywhere near the insight you need to stock a tank. Like others have said it was introduced as a rule of thumb for people new to the hobby.

 

[symon_say]

I think the "inch per gallon" rule makes a lot more sense that way... Like 12 neon tetras is no where near the same load as a foot long fish.

Anyone know where that rule originates from? Like is there any actual reasoning/science behind it other than an inch of fish-flesh per gallon seems like an ok amount? It kinda just seems like something an LFS came up with to get customers out the door when they're busy without going into a big discussion on it.

Also, as a newer person to this hobby I haven't really gotten stocking quite figured out yet. How do you more experienced folks do it? I know there's more to it than inches, like how active they are or whatever, but is there a method/system to deciding or do you just look at each case holistically?

Edit: I actually tried playing around with the numbers, using my corydoras as an example and seeing how I felt about the numbers... 2.5x.5x.5=.625, so 16 full grown corys in a 10 gallon using the cubic method, and 4 in a 10g with the linear inch method... Neither of those numbers feel quite right to me but I'd much sooner suggest 4 to a beginner (which is who you'd be telling "one inch per gallon" to anyways) rather than 16... Yikes. I think it might work better with the bigger fish but beginners (usually) don't buy giant fish.

When read about this is start measuring the fish i like with autocad (i'm and engineer) from pics i found online, and bronze cory isn't 2.5x.5x.5 the average i get was 3x1.2x.75=2.7 that gives you about 3.7 or 3~4 if you don't want to cut one of your corys. Both methods give similar results with some species but if you take into account all the species that grow 3" you'll have all kind of size in the other dimension and with linear method you'll always get the same answer with this you get different answer to each specie.

The most accurate "rule" you can do to your tank is check how your nitrates spike and see if your weekly water changes can take control of then always taking into account that most of the fish we get are not full grown. And always checking the behaviors.

 

[Katty]

Well, the fact you had to use CAD to figure that out that makes it really inaccessible to anyone to use as a "rule of thumb". The point of a rule of thumb is to be handy, the average person doesn't really have the resources or knowledge to make models of their fish and so on.

I've tried testing nitrates before but the test is bogus or something and it kept reading exactly 0 nitrates... Asked about it here and the short of it was the nitrate test is either unreliable at best and useless at worst. I don't know where that puts me... But that's not the topic at hand here anyways lol. Might go ask about that now...

Anyways, I think the cubic rule is really neat if you have the effort to calculate all that out but can get too complicated to actually use by a beginner.

 

[Tolak]

Stocking is part art, part science, and part experience that includes gut instinct, there are no "rules". Try as you might you won't be able to quantify this with any sort of consistent accuracy.

 

[Curiosity101]

As Tolak said (and I tried to say). You would only ever use the inch per gallon if you were talking to someone who was un willing to be fully helped. If they were being impatient/unwilling to listen and you want to give them a guide to avoid them over stocking and ending up with a load of unsuitable fish then you quite '1 inch per gallon', with the theory that this is more likely to give and understocked tank ie. easier to maintain.

But you always have to say it "1 inch per gallon of small bodied tropical fish"
A 12" oscar isn't a small bodied fish... therefore it doesn't fit the guideline. But most new keepers (who the inch per gal is aimed at) aren't going to be going buying an oscar cause they wont have a tank large enough. And if they do buy an oscar for a small tank despite being told it's unsuitable then it probably doesn't matter what else you tell them.

Like I said, the inch per gal guide doesn't have much place on boards like this. When we advise on stocking there are lots and lots of things to consider and each stocking thread is personalised to the persons situation. You get a 'feel' for stocking based on other peoples tanks and your own.

Things that should be taken into account when giving stocking advice:
Size of the tank
Filtration type and size
Water change routine
Size of fish (as a mass, not a length)
Types of fish - for compatability
Diet of fish
Plants
Tap water stats
Activity level/ 'personality' of fish
The ability of the keeper (New? Experienced? Spare tanks for if anything goes wrong?)

And your opinion of what does and doesn't work should be directly related to both yours and OTHER peoples tanks and what has worked. My big pet peeve is when people say 'Oh well it worked for me so you're wrong'. As they are completely oblivious to the fact that a tank is a biological system. Certain things are set in stone (high ammonia + nitrite = dead fish), but alot of things are variable so you have to base your advice on a wide set of people so you can say what is 'most' successful.

 

[markandhisfish]

i think the inch per gallon rule is pretty poor tbh. altho this persons method does at least factor in the "mass" of the fish. using the ipg as most know it would suggest tat say a 12 inch oscar would only require the same volume of water as say 12 i inch guppies , now do 12 1 inch guppies equall the mass of a single 12 inch oscar? errrrm not quite

 

[Tolak]

Everyone remembers the condensed version of that guideline. Try one inch of slim bodied fish that grows to no larger than three inches, per gallon of water. That keeps most new aquarists out of trouble during what can seem to be a steep learning curve at the beginning. It also keeps 10" oscars out of a 10 gallon tank.

 

[markandhisfish]

perhaps it would make a worthwhile pinned topic . the inch per gallon rule explained and its potential for confusion/misinterpetation?

 

[symon_say]

Everyone remembers the condensed version of that guideline. Try one inch of slim bodied fish that grows to no larger than three inches, per gallon of water. That keeps most new aquarists out of trouble during what can seem to be a steep learning curve at the beginning. It also keeps 10" oscars out of a 10 gallon tank.

That's the problem of the basic version of this "rule" most uninformed noobs, just remember the "1 inch per gallon" part of the rule, and a lot of websites have it that way to, i really don't care if people put 1 inch of fish for inch of tank in their tanks, if that's what they want.

I think that this way is a lot easy to remember, and can apply to more variety of fish, and maybe this was the way it was mend to be, but time do his trick in this matter, i'm a complete noob, and i won't even use this rule, i'll put the fish i want and make it work.

This rule is not even for average people how go to a fish store or grocery store buy a fish tank and dump all the fish he wants, this is for people like me how are noobs but want to do it right, a look for advice in forums like this and get helped for people like you, cause we want to know more about this and want to keep our fish healthy, but want a base to start stocking the fish tank.

This version is a little more difficult if you don't much about measuring but we have a lot of thing in our homes that can resemblance a fish dimension and that can be taked as a base, to do the measure, is always better to over measure that to under measure cause if you do, you'll end with a bigger stock.