Most likely, your snails are apple snails. They will do well in water with a high ph and alkalinity. You say you have very little algae, yet you are growing plants. Usually, these snails will browse the plants for alage that is too small to see. One way to make sure they are getting enough to eat is to cut a piece of zuchinni or yellow squash and just drop it in the tank. They will find it. Other things to help feed include lettuce and dead maple leaves. At lastly, the deaths could be just the quality of the stock. Check www.applesnail.net to help you identify and care for your snails.

also apple/mystery snails are often sold at the end of their lifespan when they are largest. they might be dying of old age. try buying smaller snails with a little more millage left in them.

if you are only buying snails for the algae and clean-up. you might consider ramshorn snails. they are smaller and lay eggs to replace themselves, but have never overpopulated in my tank. i can usually only find about 8-10 fingernail sized snails at any given time. they have done better in my tanks than mystery snails. i have both but like you the mystery snails seem to fade. if you are worried about over population of any snail, add a loach. they will eat any baby snail they find and keep the poulation pretty constant.

This is interesting to me because I have been keeping two snails in a classroom tank since September. They share space with some tiger barbs and a pleco.

I guess they are apple snails, based on what I've read here. They have been laying eggs in the aquarium hood and above the water line. Some eggs have hatched and I have observed babies in the tank on the glass. Then, when I look in a day or so, they are gone. Oops.

If I remove the babies, do they need to be in a heated tank? Also, for the summer, can I keep the big ones in an unheated tank?