Limescale is white, powdery build-up of calcium carbonate on in your aquarium. It can build up on your decor, heaters, filters, the plastic trim around the top of your tank and even the glass walls. Limescale is very unattractive and can be removed from your individual tank components with simple razor blades or vinegar depending on the severity of the situation.
Scrape any limescale residue directly off the side of the tank with razor blades. Thin layers should come right off and dissolve in the water.
Remove the individual tank components like the filter or heater or decor. Unplug any electronic components before removing them. Install a temporary filter for the time being.
Dampen a rag with white vinegar and dabble it on the limescale deposits stuck to your components, including your aquarium hood and the plastic trim around the tank. Let them sit for an hour. The vinegar should dissolve or at least soften the deposits.
Remove any large deposits with a razor blade and then wipe the components down with water-dampened rags.
Lemon juice works as an alternative to using white vinegar. If the calcium carbonate build-up is too thick in your tank, the only option may be to drain it, remove the fish and clean the inside with vinegar or lemon juice. You will need to set up a temporary tank for your fish while cleaning the main one.