I got one of those heating cables sold for gutters and just tossed into a similar tank with some comets and this got me through a couple of winters without incident. Our temperatures often get below 20 but rarely stay below 20 all day.

Hey there. People with above ground pools have a problem with expansion and contraction too. Their solution is to keep the water in the pool and float an inflated pillow or inner tube that keeps a hole open for expansion and contraction that could otherwise cause damage. The floater must be anchored so it is partially underwater and partially above water so it does not just wind up on top of the ice. I understand there are other tricks that work too.

Thanks albert and Sandy!

Well, I tried several ways of elevating the tarps on the stocktank, so that water/snow would drain off well, but nothing worked. So DH helped me turn them over..........what I probably should have done at the start.
I already have a heater in 2 in-ground stocktanks for the winter, and can't do any more.