On Sunday, January 3, 2016 at 8:00:19 AM UTC-6, wrote:
On Sat, 2 Jan 2016 20:09:30 -0800 (PST), bob_villain
wrote:


Question: What IS the black substance?

After changing a large number of these (a facility with 56 flush
toilets) my thoughts would be the water (and its chemical compo
sition) are breaking-down the material in the flapper. The red K
orky last longer than the black types.

It's likely mildew, but that can also cause the flapper material to
breakdown. Over time the rubber or plastic breaks down from aging
anyhow. Pouring some bleach in the tank and letting it sit overnight
might help remove the mildew.

From an aquarium site, it may be caused by cyanobacteria, blue-green algae, or black slime.