thank you for your reply,
I currently am using two tire tanks, so there black and much more insulated than the steel tanks, one is a tractor tire turned inside out with one side wall cut off, (I made my self and the second is a industrial earth mover tire I bought)
one both tire tanks, one the first I took a culvert and put it in the earth about 9 feet, (this one was not above the rim of the tank and in the course of time got filled with water,
water is at its dense about 40f degrees (why a lake does not freeze from the bottom up)
need less to say not to good of results,
the second tank the large tire tank the culvert is above the rim of the tire tanks edge, and only has air in it, (it is covered, with a tire cut in to and plywood center), it helps to keep the tank thawed but at sub zero the cold over takes the earth heat,
this year I am trying aquarium blubbers and it has some success, at least keeping the ice thin where it is bubbling,
one of the problems is moisture condensation is freezing in the air tubes and blocking the air flow, one set of lines is about 75 feet, when it is working it nearly keeps the float free,
in two of the tanks I am using electric heaters, the ones that sink to the bottom of the tank, (in years past I tried the floating ones and they did little), (trying to conserve on power here, the small tank has a 500 watt, and the medium tank has a 750 watt) they make the meter spin fast enough,
the other thing I tried this year is to put a pipe with a elbow in the one tank that has the water filled culvert in it, and dropped a aerator in the pipe to pull the warmer water up off the bottom of the culvert and move it into the drinking area of the tank, (note this tank only has one cow drinking and goats have access to it), but the goat also have the small steel tank (sheep tank) that has the 500 watt heater and a set of aerators in it, it keep the places where the aerators ice down to about 1/4 inch thick,
the large tire tank has the cattle and when they come and drink it get refilled with warmer water from the pipes in the earth,
but still will freeze up in 0F weather, (no power in that tank, not feasible to get power to it), but do have the air bubbler's and the dry culvert in the earth, but still need to go break ice daily and fork it out ever few days, before it over whelms the tank,
my goals are to go electricity free or nearly,
two tanks are in full sun light, one is only in the sun during the morning hrs,
I have considered this Idea,
http://www.ibiblio.org/farming-connection/grazing/features/builtank.htm which is similar to this commercial unit,
http://www.cobett.com/HTML/products_SBNoFloat_Main.html I have never considered this type, but I do not have any hills either,
http://agwater.okstate.edu/research-and-extension/Research/presentations/Freeze-Proof%20Stock%20Tanks.pdf may be a solar panel with a submersible pump to put it up on the panel when the temp in the panel is at a give temperature and then it would drain dry when the temp dropped,
sure would like to get the power use in a minimal use than just dumping a heating element in the tank,