Got a mantis in my 12 nano, it's still small and I intend on swapping it into my friends 55 when it gets too big. If you want a mantis and you have a reputable fish dealer by you, just ask for one. Mantis shrimp are a lot like blue ringed octopi in the fact that wholesalers get them but really can't sell them. So, they have stock but assume that most people aren't crazy enough to buy them and keep them as pets. I def agree with galma about those clowns...kiss em goodbye if you get a mantis. That said, a nano is dead on perfect for a typical mantis, and if fed a varied diet it should provide you with hours of entertainment. Mine's eaten practically everything I've dropped into the tank...including a blue ring (with no ill effects), snails, crabs, and the like. Just ask and you'll get one. Quick note: you might want to build a false 3 sided enclosure for your mantis out of mortar or sculpted live rock. The shrimp will feel secure and safe and you'll be able to observe it all the time.
