The Inch to the Gallon rule

For those of you who aren't fishkeepers, the "inch to the gallon" rule is a general rule of thumb for how many fish you can keep in your tank. According to the rule, you can have one inch of fish per gallon of tank, so if you've got a 20 gallon tank you could expect to be able to keep 10 x 2 inch fish. The idea of the rule is to make sure that the filter (which keeps the water clean) does not become overloaded.

I've been keeping tropical fish for a while now and I've discovered that while this rule is useful to newbies, on the internet it has become no more than a fish keeping forum meme. For a start, the rule is the same in the US as it is in the UK, however a US gallon is four fifths of a UK gallon, so already we see that the rule is flawed, and either all US tanks are overstocked, or all UK tanks are understocked. Considering that the rule originated in the UK, I think we can safely say that the rule is more of a rough guide than an absolute law. Yet to see how people will bark at you about how you're "abusing" your fish if you don't stick to it, you'd think it was more of an upper limit than an approximation!

The other flaw to the rule is that it does not take into account the type of filtration you are using. For instance, on my 20 gallon tropical tank I have an oversized, high power external filter capable of supporting a MUCH larger tank. As a result, I can safely keep many more fish than someone who had the same size tank but was just using under-gravel filters or even worse, internal power filters. Even so, I have in the past managed to maintain a 6 uk gallon tropical tank with an undergravel filter, with 7 fish in it! Not only did I not suffer from problems with water quality, but my fish remained very healthy indeed and are still alive today.

A lot of people maintain that the inch to the gallon rule is more to do with water aeration, and that if you exceed it then your fish will suffocate. I won't go into this because as much as anything else it's pure garbage anyway, since in order to suffocate your fish you'd have to have it so badly overstocked that they'd die of toxic water sooner than they'd run out of air. But the point is that people will make up whatever theory they want to justify the rule, because in truth a lot of them don't even know what it's based on.

So as you can probably see, this rule is far from accurate, it is just a general guide. Technically a tank is only "overstocked" if the filter cannot cope with the load, resulting in toxic water. So long as the fish are happy, you're doing everything right.

However, like I said, this rule has become a forum meme. These days you'll see photos of forumers' tanks and be hard pushed to actually spot any fish, because they've fallen into the trap of working to gain peer approval, or avoid abuse, even if it is only from people they've never even met. Unless you want to have abused hurled at you by forum zealots, it's probably best not to announce that you have an overstocked tank. Even on the practical fishkeeping forum there's a lot of misinformation, misinterpretation and zealous behaviour over this rule.

So the point of this post is to just forewarn all of you out there who are looking to find their way into the hobby; beware the forums, and beware their bizzarre fetish for the inch to the gallon rule.