I'm sure the MU test will be very accurate on that specific water sample, but bear in mind a number of the properties such a pH will vary significantly throughout the day/night. I have a test kit off amazon, but also the cheap test strips from Walmart just to see if there is an indication that needs to be looked at closer.

As far as the gypsum the OP was asking about, I haven't found a source for bulk gypsum near me either. Pelletized gypsum from the home stores would run about $225/ton and be a pain to dissolve/spray over the pond. More significantly, playing around with testing it in buckets of pond water doesn't clear the water worth a hoot. Alum is the magic stuff!

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