You can put aragonite in the filter, or add it to your substraite as a buffer. It can tint the water "white" temporarily, so don't be surprised.
If the pH of your tap water has a low alkalinity (buffering capacity) you could add baking soda to it when making water changes, start with a low dose, and increase until equilibrium is met.
A quick dropping pH can cause alkaline pH water loving fish to flash, the drop can be a build up of uric acid, which is an irritant. Although as others stated, check for other signs of a paracitic infestation.