Hard water stains?

Its in my fishtank theres a white ring around it how do I get it off?

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If it's a fresh-water tank and you're talking about at the top of the water level with the water in the tank, then try using a single-edge razor blade (if it's a glass aquarium). You can usually get a razor-blade scraping tool for aquariums at your local pet store that sells aquarium supplies.

Try scrubbing it with a micro ebrasive cleaner, like Jif cream cleaner.