I am new here but I have a 20 gallon tank with 3 platys, 2 mollies, one dalmatian and one sailfin, and a guppy. The dalmatian molly had an open sore near her gill so the petstore told me to treat the tank with marcyn cuz it was probably a bacteria infection. Well I noticed the sailfin molly can swim fine and everything but when he is still he floats with his body going upwards, any suggestions on what I should do. We've had the tank about 3 months. Thanks