I have a 55 gallon tank with a number of fish. I have an undergravel filter system and two power filters on each side to clean up the tank. My tank has been running ok for a number of months until now. I am starting to get a white foam on the top of the water. The light is even having a hard time getting through. It the foam sits to long it turns brown and gets on everything on top of the tank. I've narrowed it down to two things I think. I have reciently been adding aquarium salt to help a possibly sick catfish. Also I used to use spring water, and have switched to tap water. I have been adding a conditioner to the water... I think its called aquaclear. Now even a small bubbler in the tank with the power heads turned off, create this foam on top and I can't get rid of it. Please help.