Most of the aggressive fish like triggers, groupers and lionfish don't do real well in such a small tank. Triggers are extremely active swimmers and need more room, even the small ones. Groupers and most lionfish will out grow a small tank like that pretty fast. If you have a good top and plenty of live rock you may be able to do a snow flake, but you have maybe room for 1 more small fish and that would be it. Or you could do a mantis shrimp.
IMO, most aggressive tanks should be a 55, but 75's are even better. With the smaller tanks, there is not alot you can do with aggressive.
HTH
-Brian