I'm not a scientist and, unfortunately, I can't give you a
comprehensive explanation of nutritional stats, but I've had good luck
feeding snails on fresh romaine lettuce and spinach. Wash the greens
thoroughly before feeding to minimize pesticides and use a suction-cup
type lettuce clip to keep it stationary, or weigh it down on the
bottom with a rock. If the snails have a hard time eating it....
throw the leaves in the freezer for 10-20 minutes... this will soften
them up considerably and make them more palatable. Only dark, leafy
greens will do.... lighter ones like Iceburg lettuce are almost void
of nutrients and would just be empty roughage in your snail's diet. I
would still supplement their diet at least once a week with fish food
or a spirulina or kelp-based sinking pellet/disc. Oh, and I would cut
out feeding them things like apples and bread crusts especially...
your high mortality rate may be due to these foods. The proteins
released from the chicken shells' inside coating may also be breaking
down into phosphates & other pollutants which indirectly affect the
biological balance of your tank and may be responsible for killing
them.... a much easier and safer source of calcium would maybe be a
few clean seashells or a handful of crushed coral in a mesh bag (these
items available at most pet stores). Although be cautioned, these
items may raise your Ph and general hardness (if it happens slowly,
this shouldn't be an issue). Good luck in your snail-keeping.