Most important, we do not recommend placing any espresso machine components, including a water reservoir/water tank, into a dishwasher appliance unless specifically specified in the owners manual. Even in these cases, we do not recommend because too hot water can cause a meltdown which will not be covered under any warranty.
The water tank or water reservoir, if being used on a non-plumbed espresso machine, should be cleaned with a mild, non-abrasive dish-wash detergent at least every 2 weeks. After removing the reservoir from the espresso machine, a non-abrasive sponge can be used after a placing a small amount of dish-wash detergent and warm water in the water tank or reservoir. The sponge can also be used to clean any external part of hoses placed into the reservoir. Just remember to wipe those hoses with a clean rinsing cloth AND rinse the water tank reservoir twice filled with clean water. Dry the external part of the reservoir before placing it back into the espresso machine.
If there is a magnet float in the tank, we suggest removing the cover which holds the magnet in the compartment so the compartment and the magnetic float can also be washed. The cover usually slides upwards. A screwdriver is sometimes need to lift upwards, but it can break, so be careful.
If the espresso machine is plumbed in, the tank should be cleaned every 6-12 months dependent upon any build up of mold, fungus, dust, etc.