He wont starve. Your tank needs lots of algae to keep the snail fed, which is probably doesnt have. I would go and get algae wafers for your snail. For now, keep him in his own container. They can harbor some disease, I believe, but remain unaffected.
Bob the container int he big tank to keep the snail warm, do frequent water changes.
Did your fish die? Im sorry, just a little confused...
If so, clean out the tank with hot water and no soap. No soap, ever! Clean off the gravel, the decorations, everything. Use warm water on the filter and heater, not hot.
Let it dry out for a while.
Oh, and try blanching zucchini or cucumbers. The snail may like that more.