The problem with doing a generator on the tank is the very small amount of pressure you get from the free fall of 6' of head pressure even at 3000gph is only 54W of power even with 100% efficiency(you'll probably have 50%). To save it in batteries would be a good idea, but you'd have to look at the cost per watt of the energy and see if it's really better than just sucking from the grid to recharge batteries.
http://www.fancygaphtrn.com/calculat...wer-calculator
The easier way to cheat the system especially if you're on un-metered water is to just use your water line to your house :P. 80psi = 185' of head pressure. It would produce anytime you use water, and in a power outage, you could dump a little down the drain, haha. The solar idea would work, it's getting very cheap now to produce solar power and even in cloudy/rainy BC with some of the cheapest energy costs in the world it's becoming feasible to do it.