Hard water stains are a result of a high level of calcium carbonate, also known as limestone, in the water. These stains cling to aquarium glass as a chalky white deposit, and they are often seen when the tank is drained and then left to dry. They may also be found at the waterline of a tank that is still active. While the stains are harmless, removing them adds to the visual appeal of your aquarium.
Put on a pair of rubber gloves. While calcium carbonate is not dangerous, you may introduce bacteria to your fish that may be harmful to them.
Use a plastic scraper to scrape away at the white deposits. This method allows you to remove the offending stains without disturbing the fish.
Use a new razor blade to scrape away the white stains if a plastic scraper does not get all of them.
Pour some vinegar into a cup.
Swish an old toothbrush through the vinegar.
Scrub at the hard water stains with the toothbrush, dipping it in vinegar again as the toothbrush dries.
Allow the vinegar to sit on the hard water stains for five minutes.
Rinse the aquarium thoroughly with hot water.
Allow the aquarium to air dry.
Inspect the aquarium and repeat Steps 3 through 6 if the stains are not gone.
Never use commercial glass cleaners to clean your aquarium as they can leave a residue that is harmful to your fish.