After nearly three and half years of benign neglect, I decided to drain and replace the water in my rig. Over the years a thin black layer spread through some of the tubing. It didn't seem to be causing any problems, but I decided to clean out the three affected tubes anyway. The tubes seem to be mostly OK after cleaning, but do have some yellowing. Any idea what the substance was? Liquid was distilled water with PT-Nuke.