I would make a couple of LARGE water changes with well aged and aerated saltwater. This is often enough for the fish to recover. If the water changes do not help then you should treat the fish in quarantine with Maracyn-Two using the double dose suggested in the instructions for the first day EACH and EVERY day for 7 days. You can make a water change each day BEFORE you add the next dose of medication. Just be sure not to use raw saltwater. It is important that the water is well dissolved and aerated so you don't irritate the delicate gill tissues. You could try nitrofurazone (Furacyn by Aquatornics) instead but then you would not be able to make daily water changes. The correct dose for nitrofurazone is 30mg/gal
dosing ONLY on the first day and leaving the fish in the medicated water 3 to 5 days max. The low pH is probably what brought this on. It is important not to let the pH drop below 8.0. There are plenty of alk
and pH boosters available. I like Seachem products.