Is there any fish per gallon rule?

[Zander]

No one has mentioned it because it is ok to use as a sketchpad - but NOT a final draft.

My point was that it's at least one step above the inch of fish per gallon rule. There are many things it doesn't take into account but at least it will tell you when a fish is too large for a tank or if you have enough filtration. It's not a good source for fish compatibility though.