Oh, had another thought about using gas to heat.

What about using a pressure cooker full of water on top of the propane heater. Then, use a high temp hose connected to the steam vent to direct the steam into the fishtank or sumptank. Would be a lot less chance of contamination and heat loss. The only downfall is, how long till you would need to fill the pressure cooker with water again?

Or, if you have a old pressure cooker that can be modified, then drill two holes in it (probably the lid). Have your pump pump into the pot, and an outlet pipe flow to wherever it flowed before. As the pot gets warm, cool water from the tank flows in and then flows out the outlet warmer than it came in. Leave the steam vent with a 5/10/or 15 lb weight on it for safety even though it would never get this hot unless the pump died or you ran it so long you cooked your fish and veggies. Yummy, fish soup.