I have solar electric, so I built a 1500 watt electric heater for my 300 gallon IBC system. I have my IBC wrapped with 2" Styrofoam and the 100 gallon sump wrapped with fiberglass batts. I used a water heater element with a simple plug-in temp Control. There is a thread here with construction pointers.
I agree with David on the insulation techniques. I have done the water heater elements and they work but I had problems with leaks to the wiring. I bought a 1000 watt Paint Bucket Heater and it worked flawlessly. http://www.amazon.com/Allied-Precision-Premier-742G-Bucket/dp/B000B...
You'll need a controller http://www.amazon.com/RANCO-ETC-111000-Digital-Temperature-Control/...
This year I'm going to try to heat my GH and run a water line in front of the gas heater and bad to the tank. Electric bill was high last year.
I bought 3 surplus stock tank heaters this year that were new. I am going to try them this winter I live here in Central Texas but lost all of my tilapia last winter even here due to dropped water down into the 40's. Only down side is will need to wrap the heating elements with some kind of wire cage to keep fish off elements. Hope they work but suspect that electric bills will be pretty high. I went to Northern Hydraulics and purchased one of there 80 watt solar kits, but will have to add more panels to it to drive all of the heaters I suspect. Haven't checked out the heaters draw yet but will let everyone know if it works.
A small propane instant hot water heater, a small recirculating pump and a thermostat control for the pump. Run the in-feed from your fish tank to the pump, from the pump to the in-feed of the water heater.
The out-feed of the water heater runs back to the fish tank, if worried about scalding fish with hot water, coil some tubing or hose into the bottom of the tank and run the water through a coil of tubing before it flows out into the tank.
The thermostat control turns the recirculating pump on when the water temp drops below your preset temp. The flash hot water heater is designed to auto turn on when it senses water flow. That makes it completely automated. The pump can be a low flow low volume 12v or 24v dc pump, no need to convert your solar power to ac current to run the heater.
A small set-up like that can be built for under $300 and will heat several thousand gallons of water. A larger flash water heater can heat a LOT more.. I seen a small under sink flash heater run a 3,000 gallon tank in a single layer un-heated greenhouse in Northern Wisconsin and the guy never closed the top of his tank, almost a 1,000 sq ft greenhouse kept in the low 70's with outside high temps at -10F. He only ran through about 20lb of propane a week on a COLD week. $13 a week heat for sub zero temps is pretty cheap.
Check this one out http://community.theaquaponicsource.com/forum/topics/using-gas-heat...