Fish pond / Aquaponics as thermal mass in greenhouse?

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?

You can use IBC (intermediate bulk container) tanks as a water thermal mass.

With fish in it and heated by sun, you may also need an air pump to keep the fish happy. Try first whatever the local fish you have in nearby streams and lakes.

If you are building a high structure, why not build an aquaponics system where you circulate the water through grow beds. Or you can manually move the water to grow beds and top up the fish tank.

Water may get algae if direct sun is hitting. But you can get algae eating fish too.