I have 4 Mollie fry and I have had them for about 3 months. They aren't growing and some of them have a brown line on their fins. They are in a 20 gallon tank with 2 other Mollies, 2 plecos (I'm getting another tank for the plecos in about a month) and an angel fish. I do at least a 20% water change every other day and the filter is clean. I have an air bubbler in the tank for extra oxygen and the temperature is usually around 70 to 79° F (21 to 26° C).
Why are my Mollie fry not growing?