Originally Posted by BriNeShriMp
The rule is one inch of fish per a gallon. Any more than that, and your overstocked.
Now, that's just silly. There is no such Rule. That is simply a starting point, a rule of thumb.
Then, you consider the mass of the fish (thin or fat), the feeding habits of the fish (messy or live foods), the need for companionship (schooler or solitary), the dimensions of the tank (tall holds less than wide), the water change schedule (bigger more frequent lets you add a few fish usually), the filtration (dual or triple filtration will let you add a few fish usually), general cleanliness of the tank, the need for swimming room, the compatibility of the different fish in the tank, the layout of the tank (enough areas for territorial fish) and finally, whether you have healthy live plants in the tank.
So, for example, my 105 gallon tank has 100 true gallons in it by volume. I have 16 fish averaging 6" total length. 16 x 6 = 96, so you'd think it was perfect, eh? They are social but competitive fish, large and messy eaters. I compensate by doing frequent water changes, siphoning wastes daily, running 3 filters including a biowheel. I've added a few large fake plants to give more places to hide from aggressors. But, it is still overstocked and one day late on the water change and I see the result in increased fish aggression or fin problems.
So, please dont' quote that as Rule, it is certainly a "rule of thumb", but that is not the same as a rule.