Hi,
We have a black, grainy deposit in our toilet bowl. It increases when the toilet is flushed. What can we do?

Do you also see this deposit in the tank? Touch it and tell us if it is grainy like sand or does it mush like carbon?

Yes, check the tank. It may be as simple as cleaing the tank thoroughly. Over time deposits will build up and settle in the bottom of the tank, and when flushed, will wash down to the bowl.