Sounds like an offgassing issue maybe? Does the tap water stabilize at a lower pH, or does it stay at the low 9's after a night out of the tap? I had a weird issue in college where tap water needed to off gas overnight, since it came out of the tap at a pH in the high 8's, but would stabilize at around the 7's after being left out at night.
Another thing to consider is, if there's gravel/sand, then sometimes fish food just disintegrates into the substrate.
So many fish/plants/inverts to keep, not enough aquaria.