The glass on top of my (48 gallon) tank gets green slime in one spot only (the same spot every time). It smells like mold. It keeps coming back after I clean it. So, I took the glass top off and am using a wire top instead (like for a reptile cage). The mold or algae or whatever it is doesn't grow on the wire, but it's now growing on the plastic crossbar on top of the tank. I don't notice it in the water or on the plants or fish at all. Just on the top of the crossbar. I'm wondering if it could be caused by the light? What kind of light would be best to prevent this? I have an Oscar and a couple of koi in the tank, along with a pleco. All are over 6 inches.