What about phosphoric -phosfuric acid or how ever the h#% you spell it? I bought a 110g with some sort of residue on the back which I believe to be coraline, the tank was clean when i got it but theres just that film I cant get off. tried razor, bleach, clr, and straight muriatic acid with no luck. And it was suggested that I use phosphoric said it woul remove anything and is safe for aquarium use, they said it will actually rinse clean with water unlike muriatic that you should use baking soda to neutralize. Any thoughts? and where can you buy it?