We've had similar problems at our barn. Even though we're in the frozen north, our barn owner (I board) is short on cash and doesn't want to keep the heaters plugged in all the time.
Electric tank heaters work well, but can be expensive to run.
Last winter I made a "redneck tank insulator", and it worked really well. My pony's paddock has a typical 100 gallon Rubbermaid tank. I used four large cardboard boxes, flattened. Inside each flattened box, I put a couple of layers of bubble wrap (the kind you can buy at any office supply store). I slid each cardboard section into a heavy duty contractor plastic bag, then duct taped the four sections together around the tank tightly. It won't warm the water, but if you put warmer water into the tank, it will keep it much warmer, much longer, than an uninsulated tank.
The horses didn't look twice at the new wrapping on the tank, and on days when all the other paddocks had frozen tanks, mine was still free of ice, or had only a little on the edges.
It's not expensive, and only requires a little time and effort to assemble. Good luck!