Black deposits in toilet tank

[wa4ywm]

I thought I had posted this previously but can't find the post so forgive me if this is a repeat. Perhaps I haven't waited long enough for approval.

I have 3 toilets in the house all of which have a black deposit in the tank. The material accumulates on the right side of all the toilets for whatever reason.
It's consistency is similar to graphite lubricant. It causes black streaks in the bowl which are difficult to clean out.

I have a water softener. I do not have a water filter of any type.

A few posts I have found say this might be due to a breakdown of the rubber in the tank. I have lived in the house for 23 years and this is a recent issue - in the last 2-3 years.

Any ideas?

 

[Fitter30]

Have you looked in the tank at the water level line and under the rim of the bowl for mold?

 

[Sponsor]

 

[wa4ywm]

There appears to be a thin line of something at the water line in the tank. I can't be certain about under the rim - nothing obvious.

How does this translate to the black deposits?

I have seen photos of a mold problem in a tank but they were showing mold growing all over the walls of the tank.

Would photos help?