really depends. a good rule is 1" per every 4 to 5 gallons of water. so technically you could have 20-25" of fish in your tank. agressives need more room than community fish. for agressive i would say about 4 fish in there depending on the species, maybe you could have 5. for peacefuls, it really depends if you want larger fish(tangs/angels) or smaller fish like clowns, gobies, damsels, etc. there is really no real way to say how many fish per gallon, just give them enough room to swim and be happy. also dont overstock. later, bo