I just couldn't get the pH down far enough, so I finally stopped at the pool supply and bought some muriatic acid. I poured about a cup into the first of the sump tanks so it would filter slowly through the second tank and into the fishtank. After testing the pH 24 hours later, it was still quite a bit higher in the sump than the fishtank, so I think that means it was incorporated slowly enough. I'll test again this weekend, but I'm expecting a pH of about 7.4 and might add another half cup…
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