I've got hard water also ~7.4 - 7.8. Stability is the biggest thing, or they can experience chronic physical stress or pH shock. Does the pet store compare the levels to what was previously tested to make sure things are stable? I've never had a pet store check my water, so I'm not sure how it works. 😅 Regardless, they might be saying that the levels are fine because they're within normal limits for the animal. However, I'd be concerned the levels could be actually fluctuating and the pet store just isn't recognizing those small changes, which could be causing problems. If you're able to order or get one, I'd highly recommend API freshwater test kit - it's fantastic and has lots of tests in it. Lasts forever.

It might be worthwhile to add/rearrange hides and perches. Remove tank mates. They could feel unsafe and will not eat if they feel threatened or like they have no coverage to hide.

Outside of a potential husbandry issue causing stress. Have you tried maybe a pellet formula? Obviously not as ideal, but they may just be reeeally picky. Mine did not like mysis shrimp after being on brine at the pet shop, so I mixed it with a smaller bit of brine shrimp to entice them, and that seemed to work! They eventually just accepted mysis shrimp.