Your pH may play a role in the HITH- the GT is a south american fish preferring lower to neutral pH, if your is 8.5 and has high corresponding hardness that may be either causing, or expediting, the spread of the holes.If your fish is captive bred for many generations though it is probably adaptable to different pH, compared to fish closer to the wild.
Often the holes can heal or stop spreading if you determine the cause and fix it. You should be aiming for pH at 7ish, nitrates always under 20ppm.
Another thing I would recommend is to buy your own liquid test kit. The LFS may be telling you the water is "fine" but that means different things to different people. or they may be testing incorrectly. Better to do it yourself. API master test kit is a decent one for low price.