My [second-hand, still dry] tank has patches of residue on the inside walls. It's clear/cloudy, and I've never seen clear algae. Water beads on it as if on a greasy smear, but it is slightly rough to the touch. Soap, vinegar and scrubbing have no effect.
Any idea what it is or, better, how to get rid of it?
Any idea what it is or, better, how to get rid of it?