If you're going to raise them to near adult size before feeding, you'll need more containers to keep them in if you want a steady supply. The fish aren't going to care if they live in a storage container, as long as it's big enough and has good filtration, so consider that as an alternative to spending more money on pretty tanks.I assume the water snake will eat 2-3 fish a day and does not eat anything else? Also, if this is a wild-caught water snake, what about breeding small frogs for it? Frogs will grow quickly too and they are quite prolific. Between the fish and frogs you'd be able to have a reliable supply of food and a little variety too. If you still have to supplement what you breed to feed him with live food from a LFS or bait shop, try some crickets and different sized worms too. Wild carnivores can't afford to be picky eaters.