Need a stock tank heater for Northern winters!

I am in the market for a stock tank heater and would like to buy the right one for the setup I have/want and what my needs are.

The stock tank is 8' diameter and is a little over 700gal capacity. The winters are up here in S.Dakota so we have many frequent days of continuous cold! I would like to setup my current solar panel 240W (29.9V/8.03A) CanadianSolar (Model Type: CS6P-240P)

https://www.solarelectricsupply.com/canadian-solar-cs6p-240-solar-panels-115

So my question is, with that big of a stock tank and that current solar panel. What is the best heater to get and how should I config my battery bank?

Any help on the matter would be greatly appreciated,
Gunner

This may not be what you're looking for, but builditsolar.com has plans to build solar stock tanks that use no power and are easy to build. May give you some ideas. I'm on my phone so it's hard to make a link but this is the page.

https://www.builditsolar.com/Projects/WaterHeating/SolarHorseTank/SolarHorseTank.htm

Input
1)Electric Heating Element(100%) powered by solar panel-electric (15%)
2)CO2 Heat Pump (500% to 100%) powered by solar panel-electric (15%)
3)SOLAR PANEL-Thermal
4)Gas Powered

Storage
Stratified Hot Water Tank = max temp 195F
Stratified Hot Water Tank = 700+ gallon

Outputs
Domestic Hot Water (120F)
Radiant Floor (80F) or Radiator on Wall (140F)

I would say that battery is your 700gallon hot water tank, you just have to insulate it well and if possible stratify it.
I have seen plans for a 1)cement tank filled with 2)rigid insulation then 3)pond liner then 4)hot water.

As for how much energy your system will need each day or each month. it depends.
Once we figure out how much energy your system will be needing.
We can then figure out how much input/solar panels you need and what your storage(thermal/battery) you will need and the size heating element.

Energy Need
Domestic Hot Water = How many gallons of water per day at what temp and how much insulation of piping
Radiant Heating = Will you be using this for radiant heating and how much BTU do you currently use for heating, how insulated is your current place/etc
Tank Insulation = How much heat is being lost from your tank, how many btu

If i did my math right, that panel should raise the temp of your tank, 700 gallons/5831 lbs about a half of a degree F., in four hours of full sun. There are conductive losses, radiant losses and convection will be stripping heat off the top and sides of the tank. That is beyond my math capability!

Batteries could be employed to accumulate more energy and store power for days or weeks until needed, but even with insulation, it does not look good for keeping it against freezing.

Never tried it though, maybe someone has used electricity to freeze protect tanks that large in your region and has data on kwh vs similar conditions.

The panel would work well for powering a bubbler or gas fired heater.... or a wood pellet boiler.

We have a system installed at a horse barn and it keeps 2-3 buckets de-iced and the lights and radio on. There are days where the bucket heaters bring the battery down to low voltage shutoff, but they run them anyway.

1600w solar and 435ah 24v. I said specifically, that the system is not designed to run electric de-icers! Lack of heat is a tough one to beat.

The best solutions i have seen are locking into geothermal solar heat under the tank or a fire immersed in the tank, insulation is a given. The top should be covered.

Hmm you will not accomplish your goal with solar electric heat for sure. As an example I was just asked to design a system to keep a single bucket of water from freezing. It's draw was 80 watts it's duty cycle was over 40 percent so I had 540 watts of solar a charge controller and 2 agm batteris in the mix for 1 bucket... it was not ideal but I had to work with their existing setup.
For your situation I would probably change the tank to one that is much more narrow and more vertical. Dig it into the ground as deep as is practical and get the bottom below the frost line and insulate the sides. Add a bubbler to move the warmer water to the top and keep things moving. Moving water will freeze at a lower temp. That setup is sometimes used here and can even include a water line buried below the frost line feeding it from below... it's more work then brute btu horsepower but cheaper and more low tech reliable.
Cheers, David Baillie

Ground source heat pump? With a deep well, you could probably get close to 5 kW effective heating from a 1600W heat pump.

Yeah, a pv powered reverse cycle chiller as a heat pump driven cattle tank heater/de-icer would be great for $3500-$6k!

I think waterers become a good choice at some point. They are insulated, on demand and drain the fill pipe to maintain freeze protection of the water line. Some have heaters. There are large systems for large herds but i have no idea of the logistics or other details of even needing that much water and how it is employed.

Burying the tank or large cistern and circulating water to a smaller, insulated drinking station could work.

I thought there was a drawing here of a tank in a pit, with another stock tank on top to get the heat from below the frost line.

I like Davids idea. You could add a solar pool cover....

My uncle Fred told me that when he was growing up on the farm in Central Alberta, that they had a rather simple way to keep the stock tank free of ice.  It had a small mostly contained barrel stove with green poplar smoldering/slowly burning in it.  The barrel stove floated freely in the tank, and it was not super hot, but was consistently warm enough to keep the water from freezing as it bumped around in the tank.