Make sure before you get a puffer that you know it will go with your current set-up. It always makes me nervous when people have puffs in with other fish. All might be fine and dandy now, but when they get older it may not. There are plenty of other fish out there that you can get to eat snails, you should look into those as well.
As far as the water from the lfs...it could have diseases in it. It could have different parameters. Just be sure to not add it anymore.
Tiff