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Donovan: Welcome!
Both of the techniques you mention have been used successfully. The swimming pool industry uses muriatic acid a good deal, but as DickB mentioned elsewhere on this Board, it can be dangerous to handle compared to vinegar ... which is pretty cheap if you get it by the case of gallon bottles at a warehouse store. If you use the "fill the pond with acidic water" approach, test the pH regularly and keep adding acid as the pH increases to keep it low. You can use both together: scrub down the walls with vinegar. Rinse. Do it again. Rinse again. Then fill pond with acidic water, adding additional acid as required over the course of a week or two. Drain. Then I've seen some say they washed again with vinegar to try to neutralize even more thoroughly.
Be aware, however, that you may go a long time before a new concrete pond has no impact on pH. It can be years. But, the degree of the impact can be reduced considerably. The goal is to avoid extremes and to avoid fluctuations every time a water change occurs. [See thread re: using phosphoric acid below.]
Hope to hear how things turn out for you.
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