IMO, your LFS does not understand the dynamics of a high-tech tank and are pointing you in the wrong direction. Most fish are unaffected by pH drops even up into the 1.5 point-drop area IF the drop is caused by CO2. If it is not caused by CO2, it could be a problem but mainly as an indicator of other problems, such as osmotic shock due to more significant parameter changes. In particular, I have found Rasboras to be very robust. In fact, Rasboras tend to prefer acidic water. If it were CO2 overdosing, the first and, probably, last behavior you would see would be the fish desperately gasping for oxygen at the surface. A TDS meter (inexpensive) might be useful in determining if your water chemistry is stable.
You might also want to think back to try to recall if any of the basic environmental variables have been aberrant, such as temperature, dramatic water changes (TDS meters very useful here), ammonia issues, etc. Any of these can induce stress. Basically, I think I'd look to the disease aspect as the culprit, possibly resulting from stress and/or worsened by it, i.e.; try some of the ideas mentioned by other posters for disease treatment, although I haven't looked too deeply at your specific issues.