Only albino Sbs can live in 30s for life. Normal SBs need 40 plus gallons for life.Are you talking a standard 30 gallon (36") or a 29 gallon (30")?
Either way I think a senegal would be great, so would a male convict (better off in the 36" tank though).
Or you could go in a totally different direction and do a blue oranda goldfish... Not necessarily "monster", but when they get to be the size of a baseball I'd say they're pretty awesome