Goldfish, guppies and scuds would not give your fish the required nutrients. I feed my Grouper: frozen silversides (spearing), from the bait shop and cut smelt from the supermarket. Saltwater shrimp (most of the shrimp at the supermarket are freswater) and squid work too. You can supplement with krill soaked in garlic and selcon. There aren't really any live saltwater fish that are cheap enough or easy enough to breed, to make them a steady source of food. I prefer to hand feed frozen anyway, I think that's the coolest thing about ambush predators, having them attack and retreat from your hand offering.