I do not recommend an anenome especially if it is less than a 30 gallon setup... simply too small. They also require a mature tank. ( ie sand, rock , etc. )..and if you get a nem, good chance they will move, reproduce, and eventually sting various corals in its path.... I know because I now have 4 nems in my 34 gallon re sea max tank.
also have a read thru this lady's site:
http://www.karensroseanemones.net/
BTW, someone on canreef in Coombs was selling a redsea max 34 gallon.. you should consider that tank as opposed to making you own smaller 20 gallon one for nems.
regs
msjboy