How Big Of A Tank Do I Need?

[Mystix212]

Each 12"x12"x12" = 7.5 gallons. That tank seems fine for a small GT. My 35G is 36"x13"x18".

 

[Amaroq]

So then i have a..22.5 gallon right? I already new that but i used the 36"x12"x12"/231=22.? gallons it works the exact same way Ill most likely even just end up upgrading it on later on into a 30 breeder or something.