Instead of a Jag which might get too big for that tank, I would recommend another type of Parachromis, like a Mota (Motaguensis?) or a Freddie (Freidrichsthalli?). These are in the same family Jags, better looking in my opinion, and don't get quite as big as Jags do, but still very vicious.
Do a search on MFK for Parachromis and you'll find alot of good info. Good luck.