We have a new 25000 gallon lagoon pool with salt water chlorinator. I have 2 questions. What size natural gas heater do we need is question number one. Is 266k btu enough? Question two is what type of heat exchanger? I've read that we need a Cupro-Nickel heat exchanger and I've also read that copper is fine and it doesn't matter about the heat exchanger just the ph of the water. Please give advice as we are new pool owners and this all so new to us.