I live in North Al. and have a greenhouse that is attached to the back of my house. It is 48ftX16ft. A few months ago I got thirteen tilapia fish that really need water temps. in the 70's. They will live in the 60 degree range, but will died when water temp. gets in the low 50's. Have been working on a few solar projects to keep the water temp. up. There is app. 700 gallons of water in the long run that the fish are in. I do have a wood burner that is in the greenhouse that is used to heat our house and it never has gotten near freezing out there, but realize that this winter water temps. need to be warmer for the fish. Was wondering if there was some way of putting hot coals in metal puckets to heat up the water on really cold nights. I have one small aquarium heater in there , but with that many gallons of water it doesn't help. The temp. in the greenhouse on sunny, cold days can reach 100F easily. Oh yes, I forgot to mention that in two months my fish population has grown. I have babies app. (600 to 800) that are from 2inches to little dots. Now just keeping these fish alive till spring where they will go out side into five small ponds made just for them and will be ready to eat in the fall. Any ideas?