I'm exploring the possibility of a double-walled high tunnel (or more preferably a walipini, but my water table is high, so I might be stuck with an above-ground structure). I'm looking for a thermal mass solution that could serve multiple purposes. If it could work, I love the idea of adding an aquaponics system or maybe just a simple fish pond in the unheated but insulated grow space, which could support local fish without a heater and also help to retain solar heat in the growing space during the day, and release it at night. Is this a pipe dream? I'm in zone 6, Indiana. My motivation for wanting fish is to feed to my pet cats. I am vegetarian, but they need meat to survive. (I understand that not everyone shares my views on this, but fish are the only animals I think I could possibly slaughter. I keep chickens already and they are so amazingly smart and emotional, there's just no way.) If I could get vegetables out of the same system, that would be even better. I have access to virtually unlimited water and a solar power system. Thoughts? Suggestions?