Guess that is one nice thing about having small tanks. Just fill a bucket with cold water, drop a heater in for a couple hrs, add prime, stir with heater, wait 30 min. change water at same temp as tank works great.
If heavy metals were going to harm the fish, they'd kill the snails long before as inverts are far more sensitive to heavy metals than vertebrates (my guess is it's to do with having an efficient liver).
If the snails are alive, there's no danger from heavy metals.