We are thinking of setting something like this at our cabin.
http://energy.saving.nu/hydroenergy/small.shtml
It may be possible on your system but I'm guessing you would need to find a generator that wouldn't leach any toxins into your tank. It would also not be a permanent solution. The energy required to pump the water up to your tank would be greater than the power generated by the turbine (at least I think it would be). But it would be a great way to keep a battery charged longer during power outages.