50 gallons is Ok but just a little bit small for many species. 50 is a good candidate for an A.Aculeatus. These are an Indonesian species that many people get through Liveaquaria.com. The problem is that there is no guarantee that you will actually get one. Buying an ocpus is a bit of a crap shoot, you never really know what you are going to get because suppliers have no idea how to ID them, and even if they did, octopuses can be very hard to ID. Guess the species is one of our favorite games around here.
Bimacs are cool but they require very cold water and as of right now I am unaware of a reliable source to purchase one. That basically means if you want one you have to catch it yourself. Where are you located?
Bimacs live along the CA coast(I think) Bimacs need cold water around 60s,like Captfish said. crabs,live or frozen shrimp,clams,snails and hermit crabs when young.Avoid feeder fish due to there fat contant.Thats all I can think of for right now.
Sweet that makes this easy. Go to 'Somethings Fishy' 11730 Biscayne blvd, next to All Florida Pools on Biscayne and NE 16th AVE in N.Miami (305-982-8032). The owners name is Kay, He can order you a Hummelincki(which is basically a warm water bimac) which is perfect for your tank. He sells them for $40-$43. Feel free to tell them I sent you.