I wouldn't worry much at all. All of the above reasons seem to be spot on. Its almost impossible to see a indoor public pond thats not loaded with coins. I mean, as long as you don't use pennies for the substrate i'm sure it will be fine. For example L.L.Bean up here in Freeport, Me has a trout pond in its store that is always full of coins. Trout are actually pretty succeptable to disease, and they always seem to be in good health. So Hakunna Matata my friend!