I read this somewhere, it might have even been from this site, but I thought this was pretty good, but still general, and no real consistent "rule". I think the link at the bottom is to the site this came from, not sure
Fish per gallon rules
Small, torpedo shaped fish -4" Adult: 1" per gallon
Mid sized torpedo shaped fish 4"-10" Adult: 1" per 3 gallons
Minnows and larger Torpedo shaped fish +10": 1" per 5 gallons and long length tank
Dwarf Cichlids: 1-2" per gallon
Midsized peaceful Cichlids: 1" per 2 gallons
Midsized aggressive Cichlids: 1" per 5 gallons (exceptions with Africans)
Large sized Peaceful cichlids (including Discus): 1" per 10-15 gallons
Large sized Aggressive Cichlids: 1" per 15 gallons
Small Cats under 4": 1" per 1-2 gallons
Midsized cats 4"-8": 1" per 5 gallons
Large cats 8"-14": 1" per 10 gallons
Giant cats: +14"-3ft or greater: 1" per 20 gallons (or a lake)
Goldfish Fancies and indoor pond comets ~10": 15 gallons per fish
Koi: 20 gallons per fish
Pond Goldies: 20 gallons per fish
Carp: you need a lake at this point
Dwarf plecos and loaches ~6": 1" per 2-3 gallons
Mid sized plecos and loaches 6"-9": 1" per 5 gallons
Larger plecos and Larger loaches ~10": 1" per 10 gallons
Special fish (eels, gobies, brackish, puffers, etc.): check databases for necessary sized tanks
Just a very VERY general guide.
http://www.practicalfishkeeping.co...._calculator.php