If you are meaning what to feed 'monsters', I kept my Moray eel on a strict diet of everything. Once it is in their mouth, they don't spit it back out. AVOID AT ALL COSTS FEEDING GOLDFISH. I lost my 10" angler feeding goldfish, and my lionfish used to get all whitish and less active after eating a couple. Didn't happen when feeding them saltwater frozen (thawed, of course) fish.
For monsters (if you want to include Triggers, Groupers, Eels, Puffers, Anglers) go to your local supermarket, and you will find frozen sticklebacks, anchovies, squid, octopus, clams, etc.,..
Try it out. I recommend adding some pellets to the mix. You will find your fish growing a bit better and showing better colors with the addition of some pellets (even if you have to stuff them inside the fish you are giving).
Anglers - get them off feeders ASAP. Get yourself a thin feeding stick, and just push the meat around in the water for them, they will bite