Oh yeah, I forgot to write about the snails.
If it is a mystery snail, that is usually the name pet stores give to apple snails. Apple snails can get VERY large, sometimes as big as a golf ball or even a tennis ball. They are fun to keep though, as long as you can handle their mess. They are probably just as bad as goldfish with messing up the tank!
However, if it is an apple snails, they need both a male and a female to produce young. Typically they lay their eggs above the water line and they lay them in large sacs. However, some species of snails are hermaphroditic and will produce young alone, without a mate. These snails are typically the ones you have to watch out for. TO get rid of them you could always just crush them (I know, it sounds barbaric, but it is not so hard to do) or you could start giving them away to people who have fish who may like to eat baby snails. Also, those types of snails work well in contained decorative ponds.
Is there any way you could take a picture of it to show us?