Here's a compatible HX:
https://surpluscityliquidators.com/v..._TON_COAX_COIL
The only hx tube material more chemical resistant than cupronickel is titanium. Turbotec makes a line of titanium-plumbed coax coil heat exchangers, the pool safe line. Needless to say, these are nearly impervious to corrosion, and fall out of DIY-budget range.