I am not sure that the new "copper coins" actually contain copper. The old fashioned way to cure velvet was to add a penny or copper wool and remove it when the snails crawl out of the water. A safer way would be to use acriflavine or wunder tonic which is a pea soup of meth blue, malachite green, acriflavine and quinine, most of which will cure velvet. Some fish are very sensitive to copper.