A pH of 7.9 is a bit of a concern, especially for this species of fish. It can be one of the causes for an issue like this but unfortunately not the only. Do a few good sized water changes to see if that improves the pH some. The water changes will also elevate the pH at a more comfortable rate for the fish that the buffer. Buffers are good in a "quick fix" application but not a long term solution. The water changes will also reduce potential DOC
feeding opportunistic bacteria.
If the redness does not lessen or gets worse, you will need to treat in a QT
tank. If the fish is eating, Nitrofurazone @ 30 mg
for 5 days should do the trick. If that's not available to you let me know.