If you are keeping the greenhouse above freezing, there's no need to insulate the water tanks. Their function is to serve as heat buffers to help heat the greenhouse and they can't do that if they are insulated from it - nor can they collect heat from it when it heats up.
If you are letting the greenhouse freeze, then draining the water tanks during that time is the most reasonable option.
The actual performance of the greenhouse as built in your region will determine whether it freezes or not without supplementary heat. Consulting with other greenhouse owners or suppliers in the immediate area would be more fruitful than asking the world about it, but at a guess if you expect it to work through the winter you will need additional heat (and probably lights) in cloudy/snowy/cold western NY state - it's not 330 days of sun per year like the desert southwest.
Side note - Completely sealed (no ventilation) greenhouses have a nasty habit of overheating and cooking plants (it's either impossible or impractical to store the excess heat when the sun shines fast enough to prevent overheating.)
6mm polycarbonate "twinwall" greenhouse glazing has an R value of 1.54 to 1.62 according to various web sources. The generally more useful for direct calculation U-valve is 1/R - 0.64 to 0.62.
Ignoring roof slope, a 10x13x8 greenhouse would have 130 + 368 = 498 square feet (since you have not clarified, this is the freestanding, all glazed case.) Using the more favorable value, that means for each degree (F) below the desired temperature the outside is, you need 309 BTU/hr to maintain the difference. So, when it's -12F outside you need (45*309) = 13,905 BTU/hr to maintain the greenhouse at 33F. 33F may still be too cold form some plants, but "above freezing" is what you asked for. -12 F is the lowest temperature in the last 5 years at Buffalo airport - -20 F if you go back a bit further. That's a 4 kilowatt heater, within tolerances. 4.2 KW if we use the more pessimistic number for the glazing R-value. So you'd appear to need 3 of those 1.5KW stock tank heaters to deal with a cold night and not enough stored heat in your tanks to carry through the night.
How much stored heat could you have? You've got 4,400 lbs of water, and 1 BTU is the heat need to change the temperature of 1 lb of water 1 degree F. To provide 13,905 BTUs/hr the water drops 3.16 degrees F per hour. The water can't get too hot or your plants will die, and you don't want it to freeze, so there's a limited amount of time you can provide that for. However, there's also a "gotcha" hiding here - the heat loss calculation was done for 33F in the greenhouse - the actual temperature in the greenhouse will be higher when the water is warm, so the heat loss will also be higher, so your stored heat (water temperature) will go away faster than that until the greenhouse is down to the point that you need supplemental heating.