I think you might get some movement by convection.
Get one of those old TV Reception dish's. Put 12" mirrors in a collection around the big dish. AIm the mirrors at a ball of metal.
Water going to the ball will be heated up to as much as 2000 degrees, the cooler water in the pool will try to get up to the ball. Round and round it goes till its all the same temp.
Over night that will change and next day it starts all over again.
Another thing out of the wild blue.
Capture the Methane gas off the decaying matter. Use the Methane to power a Generator, and the Generator will drive an electric motor with a propeller to stir up the tank.