maroon clownfish

For maroons, I'd go with at least a 40. They are some of the largest clowns, and even a the 40 may be cramped once they mature.

To get a pair, you typically can start with any pair that is together and roughly the same size. The larger and more aggressive fish will turn into the female, and the smaller one will be the male.

I had a small (1.5-2") maroon and tried to pair her by buying a much smaller maroon, it didn't last long Maroons are apparantly the most difficult type of clown to pair up, to be safe I'd try to buy two from the same tank at the same time. If you're planning to keep other fish with the pair I'd go as big as you can with the tank to give them more space and reduce agression. Maroons can be nasty, mine killed several fish before I gave up and returned her. That said, I've also seen laid back maroons in tanks with other types of clowns and timid fish, causing no trouble at all.