Tanks are used to store a set volume of water on open vented systems. Be that for your hot water or your central heating. They used to be made from galvanised metal but nowadays they are made from plastic. Aswell as storing the water for your system they also allow somewhere for air pockets and expansion to go.
Ensure your tank has a lid securely fitted to stop dirt, insects and even animals getting in. These will lead to contamination and blockages.
Make sure your tank has an oppropriate jacket that is secured properly. This will prevent the water contained within from freezing and bursting the tank.
Check that the ballvalve inside is also operating properly as if its not it will either not stop water passing or not let water pass at all.
There are four pipes to check if you suspect that there is a leak.
The cold mains supply - this is where the water comes into the tank to fill it. It will be the pipe going to the balvalve/ballcock.
The coldfeed - this is the pipe from the tank to the system. Located somewhere at the bottom of the tank.
The open vent pipe - this is the pipe that enters the tank at the top. It should be the highest pipe.
The overflow pipe - this is usually made of white plastic and will go from the tank straight to outside. It allows water to flow outside if there is a problem with too much water in the tank.
If these are all found to be dry then check the tank itself.