That really depends on what you are planning on putting in it.If fish are envolved at all,Then nothing smaller that a 10 gallon would be reccomended.This is mainly because of the strain on the bio system that even one fish will cause.This strain on the system causes poor water quality in small systems.The food that you use to feed the fish will also quickly/easily pollute a small setup.....If live coral,and a few inverts are more to your liking,then I have seen sucessfull REEF tanks as small as six gallons.This can be done because live coral,and inverts have little to no affect on the bio sytem.