I have these black lines in the join areas of my wall tiles from my shower:
They are both in the lower part, where the wall meets the floor. I have similar lines near the drain:
I have tried to fix them using both an anti-mold and an anti-rust solution, but neither worked.
I should also mention that I don't have a window in the bathroom, so humidity might play a role here.
What are these black lines? How can I fix them?