Prawn is basically a big shrimp. Get wild (not farm raised) and raw (not cooked) prawns with no chemicals or preservatives. They're really inexpensive food. I could buy a pack of big prawns for $10, and it would last me 1-2 months). You could break the prawn in half when its still frozen. Feed half one day, and the other half the next day. Shell on or peeled off is up to you.
Soak it in vitamin water, such as Aquarium Fish Vitamins & Supplements: Vita-Chem Marine & Freshwater
This really helps them live for 10+ years (assuming all other basic requirements are met). I need to buy some vitamins, but haven't had the time to purchase them yet.
As you know, give them snails too for dessert (or hard shelled seafood like cockle or mussel).