For weeks I've been wondering what the gunk was filling my socks. After noticing lots of debris floating in the tank after cycling the pumps for feeds I see that this stuff looks familiar. My old tank has a canister filter which after cleaning blows similar looking stuff out of it's tubing when restarted. Sometimes I try to catch the first gallon of output in a bucked to pitch since I don't know what it is. Does anyone know what the mucous-like coating is that forms on the inside of OPAQUE tubing or PVC?