I think I just had a great idea for you! I was reading another thread about what people do during power outages for their indoor tanks. When someone mentioned hooking up a battery charger to their whatever through an inverter, then I had an idea.
I bet you can get the aerator out of large bait tanks from boats. This would run on a marine/RV battery and could not draw to much power because they would be ment to run when the alternator is not running. So maybe even something like your windmill could maybe keep up with the power draw? I dont know this for sure, but I thought I would throw out the idea for you. If you find a way to generate power that would be an easy and low draw option to check into.
Boat bait tank aerators run all the time I am pretty sure and some boats have really nice live tanks. Something else you could check into is a bilge pumps, they run on 12v dc as well, would like a DC sump pump basically. I dont know how I never thought of those before, but you should check those two things out. Could be just a way to easily circulate water if nothing else.