Put them all into one well insulated room and keep the temp in that room at 78f via two thermostatically controlled heaters?
Apparently even a small 100w aquarium heater can use about 2 1/2 kw in a 24 hour period (if it's on all the time which it won't be) so that would get expensive like ya said
This is from what I have read, I have not tested that, but I can believe it.
If you opt for the room heat, maybe try these e-heaters, they claim to run as low as 3 cents per hour, which would still be over twenty bucks a month though.
How about a single 12v boiler (if it could keep up), and the solar panels and bank of batteries, and a small 12v water pump in a closed loop utilizing copper pipe dipping into each tank, control the whole batch of tanks with just a flow valve and thermometers to monitor. Once it gets set it should stay constant.
Make sure to wire batteries in parallel and not series... hehe
That sounds like a cool setup man let us know what you decide to do
I have floor heat now, and I grew up in a solar house so I am always interested in that stuff I think it is neat.