That is a nice solenoid. I would use that in conjunction with a off the shelf $20 micro controller dev board and a couple a DC gate /120 VAC relays. Then you could program the micro controller to know last time the tank was dosed and for how long. Because you know the flow rate your could then determine the amount of water dosed per cycle and maintain a rolling average of the amount of water dosed. You would then have a nice statistic on the evaporation rate of your tank over time. Your switch algorithm could then detect abnormal dose times and override the float switch etc. Don't buy the aquarium specific micro controllers. the are limited, 3x in cost, and not nearly as fun as the generic ones! Might be a over kill but working with micro controllers is so much fun!