Can a water filter be mounted on the tank of a gravity fed system?

I live in a house with a gravity fed system. The tank in the loft feeds both the cold water and the boiler for the hot water.

I know there are water filters that are placed right before the faucet, but I was thinking if there are any that can be placed before the water enters the loft tank or just as it leaves the tank. Saving me some money and time.

Much appreciated.