A flush tank is a tank which holds fluid in reserve for flushing. The classic example of a flush tank is the tank attached to a toilet. Flush tanks allow for the storage and release of a large volume of water. In addition to being used for toilets, they are sometimes attached to equipment which needs to be cleaned by flushing, and may be used to hold reservoirs of water for dishwashers and similar types of devices.
The flush tank often fills with the assistance of a ballcock, a special type of valve. When the tank empties, the ballcock sinks, opening the valve so that water can enter the tank. As the tank fills, the ballcock rises, eventually floating up above a shutoff point and stopping the flow of water. This allows the tank to fill quickly and automatically.
There are some variations on the basic flush tank design. Some companies make dual flush tanks which allow people to choose how much water is released per flush. For light flushing tasks, only one tank would be activated, while for heavy duty situations, both tanks could be emptied. This saves water by reducing the amount of water used on light flushes, while providing enough water to fully clear a toilet after defecation. The tank can also be adjusted to change the amount of water it holds, which can be used as another water saving measure.
The most common problem which emerges with a flush tank is leakage. Over time, casings and gaskets tend to shrink and change shape, which can allow some seepage. They can also crack. Addressing leakage is important because it can generate a lot of wasted water. In addition, leaks can cause water to pool around the tank, potentially rotting the floor and causing other issues. Some signs that a flush tank is leaking include water around the tank and the sound of running water in the tank when it hasn't been flushed recently.
To repair a leaking flush tank, it will be necessary to turn the supply of water to the tank off; there is usually a shutoff valve on the floor near the tank. Then, the tank should be drained to clear it. With the tank dry, the leaking part can be removed and replaced with a new one. Most hardware stores carry basic flush tank fittings and these fittings are very generic, so it should not be hard to find one which fits a specific tank.