What sort and size of tanks?
We don't get as cold as you do, but there's a ~15x20ftx4.5ft cement reservoir on my parents
land, presumed built by the previous owners who homesteaded on the property from shortly after WWII; more than 35 years old at least, possibly more like 60.
While it's rare lately, 15-20 years ago hard freezes were common; we could skate on the ponds as often as not. This reservoir is 3/4 above ground, and ices over freezes an inch or two thick for a while even in recent mild winters. It still holds
water just fine.
A
greenhouse would definitely help prevent freezing, though how much would depend on the details of both greenhouse and climate; I don't think the barrel of water in my tiny unheated greenhouse froze last winter, but if it did no harm was done and I didn't even notice.