I have a 55 gal tank with water up to about 35 gallons and above the water line is some kind of scum or residue or SOMETHING! I don't know what it is or how to remove it. I've tried and tried to scrub it off. The water itself is super clean and it's just above the water line up to the top of the tank where this is happening. The first couple years I had this set up it didn't do that, so I'm confused. I can't scrub it off or anything..it's mainly right in front of the area where the filter is and I can't see my beautiful RES sitting on his basking plank. The only thing I can think of that I am doing any different is I use a turtle water cleaner every couple of weeks...other than that,nothing different! Any ideas? Thanks!