I went with a PEX heat exchanger. One coil, 300 feet long, one-inch nominal size. I liked the idea of having a large volume inside the tube. However, it did have a nasty plastic smell for the first three weeks of use. I’m not convinced it is any better or worse than a shorter copper coil, it’s just what I went with (3/4”x60’ copper was the same cost). I bought Uponor brand, and as I measured the inside diameter, there is 8.6 gallons of water inside. The coil was $300.
I installed four concrete blocks in the bottom of the tank, then put the PEX coil on top of it. I then spent about five hours inside the tank struggling to make a neat spiral out of the mess. The PEX is wire-tied to the concrete blocks and spacers. For spacers I used some 16-inch-long, 2-inch diameter PVC pipe. For wire I used aluminum wire sold for electric fences. These were just items I had on hand and didn’t have to go buy items, but anything could have worked.
Coil of PEX tubing inside tank, 300 feet, one-inch nominal.
The ends of the PEX pipe run out the top of the tank. This way, all connections are made outside the tank. I used a funnel and gallon jug to fill the PEX heat exchanger with water. I was worried it may have floated when I filled the tank with water. Video on installing the heat exchanger is here: https://youtu.be/WwTsWUKr5VE
Finished Heat Exchanger coil.