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As to how much ect, probably dont feed massive ones, feed smallish ones, relative to the size of the animal with some soft bodies.
It takes quite a bit for a Snail to reach full size. Also to house adult snails, the snail folks, say 1 per gallon, which I have stuck pretty close to for Adults personally. So housing that many adults would be a challenge. Babies, are tiny, and put some weight on pretty quick, so feed them when they are smaller.
Fun tip, you can double up your colonies. Keep 1 viv for your adults, your breeders, and make baby tubs. Then when the babies hatch and get some size to them, put them into a dry container. When they are not provided water, they will aestivate, (form a seal with something and sleep till water is back) they can live like this for up to 2 years.
To bring them back from this sleep, all you have to do is get them wet. The seal is made to be broken by a heavy rain. So allow them to seal up, when not immediately needed, GENTLY pull one out of aestivation when needed, and spray it with water, and feed him some food and then feed him to the Cham.
H. aspersa dont take to this as babies, but Dime sized or so, they can aestivate fine. I have been trying to get some smaller species myself, to be able to raise to adult and then aestivate extras.
Breaking the seal between the surface, may not hurt their internal seal, so you may even be able to break the seal and put them into a jar or some such, and still wake them up, I havent tried. It would be even easier to do though, and require less room for "Snail Storage"