Eric,

This programmable timer is very similar if not identical to the one that Bill Rouse has posted about using for this purpose:  http://www.ebay.com/itm/224040425984

You also need a 24vdc solenoid like this one for the timer to control the flow of freshwater:  https://www.burkert-usa.com/en/products/solenoid-valves/water-solenoid-valves/221602  

--
Mark McGovern
SM #440 Cara
Deale, MD USA