I decided to change my RedSea salt water tank to a freshwater after getting out of the salt part of the hobby. I am trying to figure out what this white scum looking floating stuff is in the last chamber where the pump pushes the water back up into the tank from my sump. I have scooped it out many times just to find it again shortly thereafter. Any ideas about what it is and if there is a solution to stop this from happening will be greatly appreciated.