In my middle tank approx 20 gallons I have 3 Platys. Originally i had five but the other two suddenly and mysteriuosly died. Fine one minute, dead the next! and the fry have also been slowly dieing off in the same manner ( they are in a large net breeder on the side of the tank. I have recently noticed that a third platy is looking extremely thin/malnourished ( though she is eating) and her gills look red, while the other two are nice and fat. It hasn't affected any other fish in the tank. I've tested the water and everything seems fine. Does anyone know what this could be? 
