A pond of appropriate size doesn't require any power, if your oxygen exchange area is large enough. Might not even need to be very deep, depending on your climate. According to my Googling, a fully-grown tilapia needs about 15 square feet of surface area in an un-aerated pond.
So, you have three ponds, all at approximately the same elevation and depth. Two for duckweed. One for fish. The fish pond is covered, and the rainwater from this cover is directed into the other two.
When it's time to do a water change, you siphon water out of the first duckweed pond, to a certain level, and throw it away. Then you siphon water out of the fish pond and into the first duckweed pond. Then you siphon water out of the second duckweed pond and into the fish pond. And finally you siphon water out of the second duckweed pond and into the first duckweed pond.
Then the duckweed ponds get filled up with rain before the next water change.
These are example water levels, showing how they change at each step: