and here's acouple of cut and pastes from US horse sites -
http://www.curlyfarm.com/photos/winter07/pages/horse_bucket_freeze.htm and
http://horselife-steve.blogspot.com/2009/12/no-more-frozen-water-tanks-your-horses.html"First off, most horses know how to break ice in a tank. So don't think you have to keep every bit out. All of our horses know to break ice, and even in lower single digits many horses will keep breaking ice through the night so there will still a muzzle-sized hole in the morning.
1. The bigger your tank, the longer it takes to freeze. So watering from a muck bucket versus a 150gallon or bigger tank is a big, big difference in how long it will stay thawed.
2. Cover as much of the surface of your tank as you can. Consider bolting a sheet of plywood to the rim around the top- you just need enough of an opening for them to get their head in, and covering the rest will help keep water from freezing as fast. Works even better if you pile dirt or even snow on top of the board, proving extra insulation.
3. You may want to drain your tank, dig some of the dirt underneath it out, set the tank in the small hole, and pile the displaced dirt around the sides. This will help use ground heat to keep the horse's water thawed. Also when you feed you can tuck your horse's hay right up against the sides of the tank to add more insulation.
4. Your best bet, if you don't have electricity, is a double-tank. Find an old leaky stock tank and place a slightly smaller rubbermaid type stock tank inside. Then insulate the space in between with whatever insulating material you can find. Insulation or Styrofoam being the best, but dirt, straw, or hay chaffe will work too. This paired with a partial cover should keep a tank from freezing over except in really extreme weather.
5. When breaking ice, actually remove the frozen chunks of ice from the stock tank. Removing ice instead of just breaking it, and replacing it with water above freezing will help cut down on ice.
5. In a pinch, harness the power of manure!! When we got hit with our first storm as horse owners we'd take a muck bucket, find the warmest, freshest piles of poop, put them in the bottom of the muck bucket, put a water bucket on top, inside the muck bucket, and then stuff the gap in the sides of the 5gal and muck bucket with hay. It worked really well! That 5gallon bucket wouldn't even form a film of ice while the 150 gallon tank had a few inches by morning"
and
"Things you can do that don't require electricity are covering most of the tank with a board, only allowing a small section of the tank exposed so the horses can stick their muzzles in to get a drink.
Place a floating object in the tank. The movement of the object along the surface can stimulate the water enough to keep it from freezing.
Insulate the tank. You can do this by building an outshell around the tank with some plywood and 2x4's. Fill the gap inbetwen the shell and the tank with foam insulation then build a lid with a small hole for the horses to drink out of.
Another way to insulate is to bury the tank below the frost line. This may or may be feasible depending on where you live and the depth of the frost line.
The idea I really like is to build a solar box around the tank. You do this in the same manner as the insulating box but with a few little changes. Firstly the tank should be painted black and a clear panel of plexiglass built into one side of the box. The clear side should be facing the southern exposure. Build a lid with a small opening for the horses to drink out of. In order to retain most of the heat seal the box with a caulking sealant"
