Toilet tank condensation is caused when the water in the tank is colder than the air surrounding it. As water collects on your toilet's tank and drips onto the floor, it can cause water stains, damage to your tile or mould on your mats. There are two general solutions to stop tank condensation: lowering the room's humidity, or insulating the wall of the tank. Try one or more of thise methods to solve your problem.
Wipe away the condensation from the toilet tank with a dry towel. Then check around the base, bowl and tank of the toilet for leaks. A leak somewhere in your toilet may be causing the system to continually add fresh, cold water to the system. Once you patch the leak, the condensation may stop.
Install an exhaust fan in the bathroom if there is not one present. The lack of air circulation may be causing conditions to be quite humid. High humidity in the room is a common cause of toilet tank condensation; especially if this condensation occurs in the winter.
Check your heating system to see if it includes a humidifier (or contact the manufacturer or installer). The humidifier on your heating system could be raising humidity levels in the bathroom and causing toilet tank condensation, especially if that condensation occurs mainly in winter. If your heating unit does contain a humidifier, see if it can be lowered.
Place a cloth toilet tank cover over your toilet's tank. It will insulate the tank from the warmer air in the bathroom and stop the condensation. Any mild condensation will be absorbed by the fabric.
Insulate the interior wall of your toilet with a toilet tank liner. Toilet tank liner kits that include everything you need are available at most large home improvement centres. The prefabricated foam liners can be cut to size and glued into a dry tank (you will have to shut off the water to your tank, empty it by flushing the toilet and wipe down the walls before you are ready to install). While Toiletology 101 recommends this method, they warn that the liners often come loose and must be reapplied every few years.