Twice a day with a wide variety of cut meat chunks. I have used:
Mussles
Clams
Squid
Scallop
Gulf Shrimp
Freshwater Prawn
Krill
"white fish" fillet: Trouts, Bass, Tilapia, Butterfish, Spot, Haddock, Pollack etc...
I avoid the oilier fishes as to keep the tank as clean as possible! The Spot and Butterfish are awsome favorites, but be certain NOT to leave uneaten chunks of these fish in the tank for over a half hour or -it- hits the fan!
I have actually seen an all tank, all species boycot of a fish called "swai". It looks a little like a catfish but the fish must think it sucks!