ramshorn snails -- how to care for them?

I got 4 ramshorn snails from someone and put them in my 15 gallon
tank (2 neon tetras, 2 zebra danios, 2 pygmy cory catfish). They are
happily slurpping up all the algae. I think they look kind of cool
and they are fun to watch. I've been reading about them on the 'net.

1. Their shells are kind of bleached. The previous owner said that
she had soft water and the snails needed the minerals for their
shells. Will adding hard water to the tank help them?

2. I've been reading that snails reproduce and can easily takeover
a tank. The best way to keep them in check is to put in a fish
who'll eat them. Loaches are the ones suggested the most often.
Loaches can get too big and they tend to be aggressive. I'm afraid
they will harm my little fish. Is there a loach who won't grow too
big and will work for my tank? I really don't want to poison the
snails or crush the babies.

Sorry for all the questions. I hope to be a competent fish keeper
someday.

Thanks in advance!
coolchinchilla