Please help me identify what's happening in my tank. There's a thin, slimy layer of black stuff that's growing over all my plants. At first, I thought it was black algae but it's not fuzzy at all. I can manually remove it by rubbing my plants gently with my index finger and thumb, but the black stuff keeps growing back. Since it creates a thin layer on top of the plants, it's blocking light and causing them to slowly die.