up here, i haven't seen a mild winter like this one in quite a while, but we still get cold..
the only way in our climate would be to heat the building that the tank is in.. big al (who hasn't been around in quite a while) had a double wall hoop house and burned, i think biodeisel, with pvc manifolds to pipe the hotwater through the ft's.. i think the lowest temps he saw were in the 40's..
in fl? i'd insulate the hell our of everything, gb's, sump, ft and cover the ft with plexi backed with foam with a hatch of some sort.. i'd even think about putting 1/2" foam board over the top of my gb's and cut holes for plants
i think adding air in a system that's "tight" would be imperative and this could also be a way to bring some warmth into the ft if you can put the air pump in a warmer area, or use a small heater in a small enclosed space (with appropriate ventilation) to preheat the air before going into the ft..