paul:
Welcome to the forum. I've used a 300 watt aquarium heater in a 100 gallon stock tank that was sunk into the ground for use as a small "pond". That worked to keep it from freezing too thick and the fish in it survived the winter. I had a 300 gallon stock tank in the ground as well, and 3 of those aquarium heaters did the same thing for it. So, 300-400 watts per 100 gallons is my rule of thumb.
Those floating stock tank heaters will do the same thing, but the one that I have is 1500 watts. I thought it was overkill for what I had since the tanks were sunk in the ground and somewhat protected.
My stock tanks are the composite tanks from Rubbermaid, and I just laid the aquarium heaters on the bottom of the tanks.
Dave, de-icing means turning hard water back to soft water.
