My convict is an absolute horse, but if I even skip one day of feeding him (and feeding him WELL), I notice him start to look skinny. At the moment, he's about 6" long and nearly and inch thick, so I can only tell you what he's been eating.
Mine grazes some very short hair algae patches on the back glass, picks a little at film algae on the glass and pumps, but mostly goes nuts for anything leafy I put in the tank. He LOVES nori (sushi sheets from the grocery store...MUCH cheaper than buying nori at the LFS and exactly the same stuff), will eat green leaf lettuce almost as quickly as nori, and will pick at broccoli over the course of a couple days. Just recently he's started taking the mixture of pelletized food that the other fish usually get. He tends to leave shrimp alone when I feed chopped shrimp once or twice a week.
I'd try the garlic thing, as I use it once a week or so and it does seem to stimulate feeding response. I just got a little jar of dehydrated garlic from the grocery store, grind it up to a powder, put it in a cup with the food and some water, and let it soak for 10 or 15 min before feeding. The fish go ballistic for the food.
