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And, another oops! I'm editing a semi-standard answer into your main reply, but posted it before I finished. Here's the bit you need NOW:
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Obviously, you need to get your pH up. With most kits or strips, a test result of 6.8 does NOT mean your pH is 6.8; it means it is 6.8 OR LOWER. It could be 4.8!
Your NORMAL borax dose is below. But you need to add borax FOUR at time or soda ash TWO boxes at a time.
Soda ash (regular pH UP) (typo corrected!) is cheaper, but raises carbonate alkalinity which you may not need. Borax has no undesired side effects, it's preferable if you are not on a tight budget.
Regardless, add the chemicals SLOWLY to a skimmer while the pump is ON and water is FLOWING through the skimmer.
Leave the PUMP ON. Retest pH after 6 to 8 hours. Continue to use the non-standard doses till your pH is ABOVE 6.8.
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