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Dr Chemical

The simple solution is hydrochloric acid if it’s in your toilet bowl. Harpic used to make a product that was thickened hydrochloric acid but they’ve taken it off the market now. You can use as much as you want on ceramic – just tip it in and let it do the job.
If it’s in a sink where the acid might dissolve metallic components, go to Bunnings and get a product called Gelimac – it’s oxalic acid, and will do the job nicely (although not as quickly as the hydrochloric)