My mother in law bought several platties and mollies from a local fish store. These new fish all have clamped fins and are behaving listlessly. Sometimes they hover at the to of the tank, other times they'll just sit near the bottom of the tank. The fish store told me they probably have intestinal parasites and the condition is incurable. We have 5 neons and 2 pearl danios in the tank with these sick fish. The neons and danios are about 2 years old. What can I do? I test and change the water on a regular basis.