i've been cleaning my fishs tank each week. It is a plastic 2.5 gallon tank with just one fish, a heater, and a peace lily sitting at the top of the water. I've had this same set up for over a year but lately, this weird mucousy film has been developing at the top of the water. You can pretty much pick it up off the top of the water, its super gross. I cant get a picture of it because its clear but i want to know why this keeps developing. I used to use a water clarifier and i thought that maybe it was the reason this mucous keeps developing but the last time i cleaned the tank i didn't put any in and its still there. Any ideas?