oil hot water heater--WANT TO CONVERT to pool heater

i would like to use a aosmith duraclad oil hot water heater to heat my pool.
i am concerned that the iron in it will be acquired by the plaster in the pool and will stain the walls. perhaps the water heater has glass liner, but won't the heat exchanger contain iron ?
alternatively, perhaps i can open it, add a stainless or aluminum heat exchanger and close it back up (with a torch). i can get the exchanger free from a friend steamfitter, but need to rent the labor with a torch. runtime pressure is 7-15lbs.
any advice ? i got the water heater free (as in free beer) from craigslist. it will be a great project, if i can get a little practical advice.
135,000BTU from 1ga and 30,000 ga. from 42F to 75F will cost about $120-150. That's good for buying 3 months early usage...along with the bubble wrap i already have to keep down the evaporation. this equates to $1/day of warmth usage; less inefficiency.
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