Heaters should work. How big is the tank? Are the heaters hot when you touch them?
I've never tried it but I heard of a milk jug half full of salt water so it floats and doesn't freeze. Then it keeps a opening through the top and the horses push it down to drink. Not sure if it works or not tho just heard of it!
I would try heat tape if you haven't already. It tends to work for me pretty well.
Put a tennis ball in there and when it freezes it will break the ice up
Adding apple cider vinegar might help if combined with the heaters. But it will only lower the freezing point by a few degrees. You could also try only adding as much water as they drink so they drink it before it freezes but this is quite risky if you cannot check the water often. Half enclosing the water in some sort of insulated room or she'd might also help.