Large bag of Daphnia (probably D. magna) to start your own culture of live foods.  Daphnia are highly nutritious and their movements are very attractive to finicky eaters.  A must have for breeding small, advanced-level fish species like killifish, anabantoids, dwarf cichlids, etc.  Daphnia feed on cloudy-water microbes like bacteria and algae and can be easily cultured year-round inside, or mass-produced in stock tanks outside during the warmer months.