I use a Plast-O-matic THP with a W20 solenoid coil, 418$ from
www.plastomatic.com. There are other vendors/models of this type of valve (look up "actuated solenoid valves" with Google), but the prices are similar, and the Plast-O-matics product seemed (to me) to be the best match for an aquarium environment. It has worked unfailingly for about 1.5 years now.
Ken