I would get a Blue or Gold Rhom. If you can ever upgrade to a larger tank I would get an Aba aba; not the most colorful fish but possibly the meanest. They grow fairly slowly so a 125 would last quite a while.
a honduran male jag with green and blue fins they grow huge and have a really cool head shape imho. i have a 110 that has two jags, they are really intelligent acting and have personalities, it seems
Get a DOVII! Extremely agressive, active, gorgeous, and very predatory. Also much rarer than the other parachromis because most people cant accomodate them
He's in TN Snakehead are illegal or else that'd be a great idea.
What about a Wolffish? Perhaps go Brackish and get a small Moray?
Perhaps a small Tigerfish?