Background: I'm in the final steps of setting up an aquaponics greenhouse after running a smallish indoor system for several years. While our indoor system is pretty conventional, for the greenhouse I wanted to marry aquaponics and "Summer Tubbing"-style fish breeding to create something more in line with what I've been learning about permaculture. (I plan to breed threatened species of nano fish rather than food species, both because it's something I really enjoy doing and because their lower bioload makes low-tech systems easier.) I'm still in the early stages of cycling the tanks (6 200gal IBC sections) and adding some plants, and I'm currently using electric heaters while I work on better solutions. I'm at 6500ft altitude and have no shade, so I feel pretty sure I won't need to run the electric heaters for long.
My first experiment was to take a piece of scrap foam, cut a hole in it for a 3" net cup of hydroton, float it in one of the tubs, and add some kale seeds. It's been a week so far, and they seem to be sprouting pretty well. I figured I'd keep an eye on them, thinning when necessary, with a goal of seeing if I can grow a plant all the way to maturity without capsizing or root rot.
The improvement I thought of yesterday but haven't had the chance to try yet is to take some dark-colored soda cans, cut them up so that I have a rectangle of thin aluminum, and then attach them to the DWC foam with the dark side facing up and one end going a few inches into the water. (Ideally I'd go as far down into the water as I could to promote water circulation, but I haven't figured out how to do that with available materials so far.) My hope is that during the day, the dark aluminum will collect heat and shuttle it into the water, and at night, the aluminum will gently radiate heat from the water to help keep the plants warm.
Has anyone tried something similar? Am I overestimating the heat conductivity of aluminum cans and setting myself up to cook my plants? Any suggestions on foam sources or substitutes that don't put extra styrofoam into the world?