I've Been keeping saltwater for years now. Anything less than a 55 and your parameters will be extremely difficult to maintain. Your salinity, PH, calcium, alk , etc will swing wildly due to small volume of water. I highly recommend at least 40 G to start. If you take all the rock, sands into account 40 G isn't 40. It's way less than that. Do you know anyone like a friend or coworker that's into SW? If so ask ton of questions before you start your own tank. SW fish are alot harder to take care of. Theyre extremely prone to saltwater ich (not the same FW ich) and its the leading cause of death along with cyanide poisoning. I suggest you steer clear of SW unless you have the time and dedication to take care of your fish. I dont mean to discourage you but that's the truth. There's a reason why there are more FW hobbyist than SW.