I bought two 24V DC Solenoid valves on Ebay, one NO the other NC. The NO valve is in the saltwater feed line and the NC Valve replaces the manual flush valve and feeds fresh water through a TEE into the filter housing. The bottom of the 3 connectors on the TEE has the NO solenoid for saltwater supply connected to it.
The solenoid valves are wired to a small 24VDC digital timer, also from Ebay which is ,mounted in the cupboard space above the sink. The timer has 3 functions OFF, MANUAL, AUTO. When away from the boat, we ensure the water tank is full and the timer is set to Auto to flush for two minutes at 1000 Mon, Wed, Friday. It only uses about 4-5 litres each flush.
The whole set up probably cost less than $100.