On one of my tanks (newt tank to be exact, 20 Long) their is a light film collecting ont he top. Almost looks like oil on the top of the water. There is plenty of surface agitation but I am not sure what is causing this. I am trying to do a 1 Gallon water change every day to see if that helps. I am doing this because I think it is caused by an excess of AmQuel+ (de-clorinator, nitrate, nitrite, ammonia remover). Any other ideas or ways to solve it?