I know it sound silly, but a nicely planted 125g tank full of small tetras would be awesome. In a tank that size you could keep a dozen marbled or silver hatchetfish, a dozen pygmy panda corys. a 1/2 dozen skunk corys, and 30 each lemon tetras, glolight tetras, neons, cardinals, gold tetras, rummynose tetras, and head and tail light tetras, as well as a couple of twig catfish and two-rayed bajo cats as well. Not a monster in it but an attractive tank none the less.