When I was a kid my dad, brother and I built a 20000 gal in ground pool at our house in Ohio. The pool pump tee's to 200' of black 1" ground water poly tubing that was coiled up to about 6' x 6'. We built a wood box around it and placed 1/4" of Plexiglas over the top and sealed it tight. At the tee we had a valve that we opened slightly when we wanted heat, or closed to bypass the heater. The loop tee when back into the line just before the jets. Worked real well. We kept a blanket on the pool at night, and had warm water an extra 3 or 4 weeks a year, and had the pool up to 98 F till the algae caught up with us. It was simple, cheap and set in the yard just down hill a little from the pool. Ah the good old days. Dad dries corn now with a 18000 gal propane tank so he has a big ugly gas heater next to the pool now. :-(