A few thoughts, but first a question, does the tank need to be exposed to light? If not, you might try wrapping it with reflective bubble insulation and put an aquarium heater under it.
Discussion
well... first, the answer is No.
I have some lucky goldfish out in the pond to contribute the minerals and nutrients to the medium.
second. I need heat in the shed (air) to support the above medium growth.
i have side lights for the tank to sustain the fish and maybe a couple of water plants.
still researching