My first two years doing Aquaponics, I had my structure over my main Aquaponic system which provided handy support for plastic to turn it essentially into a greenhouse.
However, as many know, last spring code enforcement came around and said I had to take it down unless I got a building permeit for it (to get a building permit requires drawings stamped by an architect or engineer.) I could have made drawings but I never figured out how to get the proper stamps. So, the "greenhouse" came down as did my outdoor kitchen cover.
Last winter was quite severe for us here in Central Florida and the few tilapia I had left when the really cold hit did not survive. I have not replaced the tilapia and only have catfish this winter. They will survive nearly freezing water as long as they are big enough not to starve to death.
Now I like the cool weather crops like broccoli, cabbage, kale and such which will probably do better for me without the heating of the greenhouse but I may have to kiss the banana and papaya good bye, at least I'll have some fruit to remember them by.
I am a big supporter of growing fish appropriate to your climate. Flood and drain media is a big reason for this. Flood and drain gravel beds make remarkable heat exchangers so if the air is cold and you are flooding/draining your gravel beds, they will do an great job at bringing your water temperature closer to whatever the air temperature is. The water and gravel might make very good thermal mass in a greenhouse but if the air gets remarkably cold in that greenhouse overnight, flooding and draining your grow beds during that time period will chill the water. Even shutting off the pumps overnight will only postpone the chill till when ever you do the first cycle in the morning though this will mitigate the effects to an extent. Adding water and running the beds as constant flood through the coldest spells of winter might actually be more effective at maintaining water temperature and since most fish slow down a bit during the cold times, the reduction in aeration and filtration by running constant flood through the chill may not be an issue.
So as I go into the cold, here is a picture of what the system looked like before hand.