If you read threw the stickied topic about the 1" per gallon rule thread.. the short answer is every species is difference so an over-all general formula is just going to give you the basics of getting started.. imo take the adult fishes size.. 1.5x= the depth and 3x=length 1x=height is a good start... but for fish that are more active will need more room, fish less active would be content with less.. some fish are wider, some narrower... so a 12" adult average sized fish.. will need a 75 for life to be happy in general. Most here will argue that a 125 would be better and a 150 best depending on the species, and in some cases filtration. Oscars imo being the best example here. I've seen Oscars content in a 75 for life.. and some that where almost 18" and needed that 150. at least these are my own oppinions on the matter, and bigger is always better in reguards to water volume.