That gives me horror flashbacks to my uni days. We gave our messiest housemate his own loo on the bottom floor, and when we left after two years it was stained completely black. Took about four bottles of bleach to tackle the bugger as wellThat’s definitely staining in the toilet & not Coke as I suspected earlier. When she dropped the toilet bomb In the water that splashed up was clear so whatever it is is caked into the bottom of the toilet.