How do I heat a greenhouse for the winter?
Inside the greenhouse is an Aquaponic System, with live fish and plants. The greenhouse is a 10'X12' aluminum, with polycarbonate panels. I can't have anything that will kill fish, plants or melt the panels. It is not the least "air tight".
My first though is heat the water and it will release that heat into the air keeping it at a reasonable level.
If that isn't enough heat for you, you might consider a small electric resistance type heater, radiant heat, or even a hydronic based blower.
Gas, kerosene & other type of combustion space heaters should generally be avoided in this type of environment.