Cold water tank problem.....?

My cold water tank is inside a cupboard along with the boiler and a few shelves. Recently i have notice more and more water (condesation) forming on the outside of the tank and now the carpet is wet out the cupboard door and the floor is really damp and mouldy. I don't use the space for anything else and rarely go in there. Any ideas how i can reduce the amount of water(condensation)? I have a dehumidifer and always have windows open.