[Ihor]

When I fill my used glass coffee pot with well water from my kitchen tap to wash it the water turns translucent black. What is the chemical reaction that causes this and what element/ compound is the cause?

[rolnor]

I guess you mean something comming from coffe residues? Maybe tannines?
https://roastmeisterscoffee.wordpress.com/2012/02/03/are-there-tannins-in-coffee/