To remove limescale stains from a toilet, apply a gel or bleach product made specifically for hard water stains, covering the deposits and squirting under the toilet rim. Let the cleanser sit for approximately 30 minutes before flushing the toilet.
If any stains remain, scrub them with a pumice stone or use a heavy-duty cleanser, such as Borax or water-soluble trisodium phosphate. The longer the cleanser stands and soaks into the stain, the easier it is to remove.
The cause of limescale is above-average mineral deposits in piped water, making it "hard" water as opposed to rainwater, which has few impurities. Two long-term solutions to limescale problems are the addition of a water conditioner and a water softener.