Does anyone know what this is in my toilet? (Image attached)

[Landlord21]

Recently took on a new rental property and when I first moved in there was a large black mark at the bottom of the toilet. I put bleach down the toilet as cleaning wasn't removing it and left it overnight and the next day the mark was still there but it had turned grey instead of black. I've done this about 5 times now and it's not getting any better. No amount of scrubbing will remove it. When I close the bathroom door and reopen it a bit later on there is also a horrible sewage smell. Does anyone know what this is and how I can get rid of it?

I know this sounds crazy but have you tried two litres of coke pour it down and leave it overnight , it looks like limescale

Black harpic is your friend here

Don't bother with bleach. You want the black bottle of harpic. If that doesn't work nothing will.

Black Harpic. Leave it overnight.

Another vote for black Harpic. It is limescale so solid scale build up. Because I am extra I remove the water and then use black Harpic but I live in a hard water area. I do this in the day time and go back to give it a little scrub every hour or so rather than leaving it overnight. Yours is teeny, so an overnight soak should work fine.

Bleach won't remove it, just bleach the colour out of it. You can get Harpic limescale remover in a black bottle at pretty much every supermarket.

Ah happy days! I will be straight to get that! Would the limescale be what's causing the odour as well?

It has eyes- it’s alive!!

If you have a mop take the head off... Use the hard end to chip any last bits off...
Use the full flush for a few weeks if there are 2 flush options.

I think someone has damaged it with caustic soda or something and there is also a blockage

Black Harpic is very good, but if that doesn’t shift it you need kilrock or viakal

There doesn't seem to be a blockage, it flushes fine it's just a flat mark at the bottom of the bowl.

White vinegar will also remove limescale. I describe the kettle first then to it down the loo to descale that.

I'd put on gloves and feel the area to see if the enamel has actually been scratched away, or if its a build up of limescale that could be cleaned off? If the enamel/porcelain has been scratched off, then the only fix AFAIK if a new toilet bowl. Is it on the inventory? If it is scratched porcelain I'd be taking pics and informing the LL. If its just limescale build up, I'd push the water out using a plunger and trying viakal/vinegar/toilet limescale tablets etc.

Use a pumice stone on a stick - great for removing limescale.
https://amzn.eu/d/eSmPGNv

It wasn't in the inventory but I listed it and took pictures. This is what it looked like when I first moved in before I tried to bleach it and the colour came out of it.

The black harpic worked perfectly!! Thank you all so much!