What should the temperature be for a blue mystery snail?

I have 3 goldfish, and Ive tried and failed twice to keep a mystery snail in there. On my second try, i noticed a spec on the snail when I got it, it turned out to be a baby snail, so I put the 2 snails in a little tank. The big one died, but I still have the little one. I want to try to put the little snail into the fish tank when he gets big enough, what can I do to keep him alive? What went wrong with the other one?|||goldfish and koi eat snails