Local ponder here did his pond in concrete, similar to Jim, I think it was
4" thick. I remember them using muratic acid soaks, 1 gallon/1000 gallon
water. Seems you want to keep it acidic for at least a week, by adding more
acid as needed, drain, rinse, fill, wait, and test pH. If 7.5 or less after
a week or so, I'd think you'd be good to go. ~ jan
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Zone 7a, SE Washington State
Ponds: www.jjspond.us