I have some backflow appearing in my toilet. That means that after some time, the bottom of the water bowl will slowly start to darken I have tested with disclosing tablets and coloured limescale tablets and you can see what I mean from the photo. It tends to vary in severity. A plumber from homeserve came and said it was more of a communal issue (I am in an apartment block) and the management of the building says it is internal. I used a small snake brush in the toilet and it didn't do much so u-bend of the toilet seems fine I think the issue could be with the larger waste pipe at the back (picture attached) that the back of the toilet connects to. It is like the water is settling there a bit when the toilet is being flushed and then slowly moves back into the toilet. Could it even be a be a build up of toilet paper that is soaking up some of the water or is blocking waste? And then there is a creep backwards. Any help/recommendations would be much appreciated.
You could be right about the waste holding back. I would try to dislodge it with a bucket(s) of hot water and a plunger in the pan.