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March 31st, 2008
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| | Fish Master
| The inch per gallon rule - what it is and how it works.
Hi!
Well, I thought this would make useful reference for the 'Aquarium stocking questions' forum, to help explain the general rule of thumb we tend to use here in some detail, and explain to people who believe that this rule is 'rubbish' that it is a useful tool when stocking a tank.
Basically, the inch per gal rule is a quick and easy way of estimating how many fish you can have in a tank. Say, we have a 10gal tank. We want to stock it using to inch per gallon rule. First, we must take into account that this is only a general guide. It does not account for the shape of the fish (A kuhli loach would only count as 2-2.5" as it is long and thin, even though it's actually more like 3.5-4") and the minimum tank size for a fish. Just because a Bristlenose pleco is 4" - it doesn't mean it should be kept in a 10gal as its absolute minimum tank size is 20gal. Secondly, we must remember this rule accounts for the adult size of the fish. Even though that Clown loach is 3" now - their full grown size is more like 12"! We have to account for the Clown loach being 12" as it will be in the future - not 3" as it is now. This rule way originally designed for small tetras, like Neons. However, if we take these 2 things into account, it can be used as an effective guide. So, we've learnt that, we must now learn how to use the rule.
Using the rule of thumb is easy - work out how many inches your tank will hold - our 10gal will hold up to 10 inches without encountering problems quite happily. However, since the 10gal is such a small tank we'll say it has space for 8" of fish - as it is much harder to create a balanced environment in a 10gal. Now, we must work out what fish we want. Lets see... What about a Male and a Female betta? A Betta is 2.5" long - 2.5 + 2.5 = 5"! That will leave plenty of space to swim. Wait - of course that won't work! Because a male and female Betta are unlikely to get along - we can't put the together even though they fit in the tank. Back to the drawing board! Hey, Betats and Otos are usually compatible. Let's put 3 Oto's and 1 Betta in there! This will work, because Oto = 2", (3x2) = 2.5 = 8.5! So, it's a little bit more fish than we said, but because the tank is 10gal there is still some space. Wait, what's this? We can only do water changes every other week on our schedule? That means we must treat is as a much smaller tank, lets pretend it's a 5gal. I think if we are to be sensible here, we must only get 1 Betta for this tank. Otherwise, the waste will build up and the tank may go through a mini-cycle, and Nitrates may get out of control! So, just 1 Betta.
Mind you, if we could do weekly water changes, and the tank was a 29gal, we could get away with around 2-3" above this rule, as it is only a general rule of thumb.
Hey, but look at that Oscar over there! He is cool. Wait, he looks like he'd be comfortable in a 29gal tank because he's about 12" long full grown. Wait, with large fish we must use a different rule! It's the 1 cubic inch per gallon rule. So, we count fish like that 2" wide by 4" high by 12" long over there as 96". He'll need something like a 100gal tank. Maybe not the Oscar then! The inch per gallon doesn't work on big fish like Oscars or goldfish.
As we have learnt here, the inch per gallon rule is only a general guide, and must be used in the correct way, or it will be irrelevant as many aquarists will have you believe. It also does not work on big fish. It would be better known as the inch per gallon 'guideline' - as that is essentially what it is.
Last edited by Blub; April 4th, 2008 at 01:28 PM.
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March 31st, 2008
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| | Fish Mentor
| This only works with small fish.
It does not take into account the biomass of fish, only their length. | |
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March 31st, 2008
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| | Fish Mentor
| As Dino stated you have to use a different "rule" with fish with a large bio mass....such as bala sharks, or oscars.....instead of one inch per gallon, it is more like 1 cubic inch per gallon....so if the fish is 2" thick x 4" deep x 12' long it would be counted like 96" thus would need to be housed by itself in a 100 + gallon tank!
This is why gold fish have to have such a large tank, and so many people say you have to have a huge tank for oscars, or any other fish that grows up to 12 or so....they are thick as well as long.
Also keep in mind that several of the larger fish also need to be kept in groups, and ample space is needed for the group. | |
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April 3rd, 2008
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| | Moderator
| Ugh...Bold, centered text. Difficult for an autistic guy to read. 
I would suggest the word "guide" instead of "rule". There are so many exceptions to this that it can't really be considered a rule. | |
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April 3rd, 2008
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| | Moderator
| Stocking a tank depends on so many things. Size, biomass, needs(schooling or non ) etc. This "guideline" works pretty well for neons but anything bigger and it all flies out the window, many many variables start coming into play.
Carol | |
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April 4th, 2008
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| | Fish Master
| Quote:
Originally Posted by sirdarksol  Ugh...Bold, centered text. Difficult for an autistic guy to read.
I would suggest the word "guide" instead of "rule". There are so many exceptions to this that it can't really be considered a rule. | Hi!
I'm autistic and I can read it. 
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I'll put it in now.