There are a couple of green methods but need some real life testing. One is to store heat in a tank that is heated from solar piping. Need to experiment with solution in the tank that stores the heat. Something with a high specific heat. You know, potatoes keep their heat for a long time, so I was thinking of a starch solution, with a preservative like formaldahyde to keep it from going bad. You would use pipes in that solution to get the heat back out.
Second method could be car battery with solar panel. My thinking is that in my area, I need at most 6 hours per day for night time heating without recharging from the sun. The big 1000 watt heaters are recommended for 160+ gallon tanks. So, that is 1kW*hr * 6 hours or 6kWHrs. At 110V that's 54amp hours of charge. Let's triple that for losses in the inverter and others, and we get about 160 Amp hours of charge. This is within the charge holding capability of a battery. Problem is the charging. Even with the high end solar panels that seems to be 3 large panels at 10 hours of light to charge up. Maybe solar charging in addition to mains charging.
Anyway, needs some field work and testing....
I just use aquarium heaters. 200 watts heats 50ish gallons, so you just go from there. Easy, readily available, but definitely not the best way to heat a large body of water. If I could wave a wand and install whatever I wanted I would install a geo-thermal system in my greenhouse and heat the water using heated water pumped in from the earth...but that is pretty pricey!
Yeah...I'm in Virginia and I'm expecting some really cool temps this winter. I have basically the same set up as you describe,TCLynx. One diffeence is my tanks will be insulated with 2 1/4 inch rigid foam with a top of the same mterial. The grow beds are constructed of the same material and I'm making hoop covers with clear plastic for each one. From what I understand, smaller fish will weater the colder temps better than grown fish. So the object with the insulation and hoop tops is to contain the warm water to the least amount of airspace possible. Then, to harvest the Tilapia before wnter sets in ( filet and freeze) and start my growout through winter with fingerlings.
Well, actually, I was talking about exactly what I described in my post.
However, I do remember one year a long time ago I had just moved into a lakehouse on a beautiful pristine lake. My second night there I caught a catfish fishing from my dock. Knowing I had someone coming to visit that weekend I took the catfish, whole and put it in the freezer. It was in there overnight. The next day, as a joke, I filled the bathtub and put the frozen 5 lb catfish in the tub, knowing my visitor would want to shower after the trip. After my guest arrived, we talked over some coffee for about an hour and then, as I expected, requested the shower. I waited for the scream and got exactly what I wanted....I rushed in as though I was surprised and wound up being more surprised than my guest. The catfish was alive and well splashing around wanting out of the tub!
No actually, fingerlings don't necessarily survive cold better. Now this all depends on what kind of cold you are talking about and what kind of fish.