lucky you! here it is iron in the water that feeds harmless bacteria.
if we take the faucet off or do any other kind of work on the pipes the water that comes out of the pipes is black for a few minutes until it will run clear again. i think that is also iron fed bacteria lining the pipes a little. as it doesn't plug the pipes it's not a problem i worry about.
as to why to do it once a year instead of buying a new flapper each time? seems rather silly to me to pay money for a new flapper when a few minutes of time and a bit of vaseline and it's done. but to each their own i guess...
songbird