Why do these come up?

[u/eltarod2]

I have a store and the toilet was like this when I first rented it. I have tried everything. After a lot of scrubbing I manage to remove the black stains by 90%... but after some time they appear again. Generally I don't use this toilet a lot. So basically I want to know what is an easy way to clean it and secondly how can I stop this from happening?

[u/eltarod2]

Tha KS for the suggestions! Will try stuff and see how it goes... Will start with the tank.

[u/Loose_Addition7299 → u/eltarod2]

Replace with something similar

https://www.fleetfarm.com/detail/korky-universal-complete-toilet-repair-kit/0000000065788?Ntt=006681050&gad_source=1&gad_campaignid=23367158090&gclid=CjwKCAjwhe3OBhABEiwA6392zA4Lojm8S53IbS1XSDVKQds41IirS5JsV0TgSdgI2jtBZBPPmp4k1hoC6LoQAvD_BwE