What I see is one red area on the fin, and that appears to be a netting injury. If that is what you are seeing and asking about, give it time. These types of issues will generally fix themselves, given time and good water.
Speaking of water, I don't know what "Gh, kh, good" means. They are always given numbers, like 80ppm or 150ppm, or 4 drops, 4 degrees, or some other number. Good doesn't help us help to get the best water. With a pH of 7.4, is that morning, evening, since I am most familiar with the waters from limestone deposits that have a reasonable KH, the pH will generally be closer to 8.3 if the KH is high enough to provide stability. If you are using test strips, they are notorious for being poor substitutes for drop type test kits, nearly unreadable for some of the tests, and if they have age, just plain wrong. I would recommend the API drop type test kits for Ammonia, Nitrite, high range pH, Nitrate, and KH, with KH being the most important of these going forward, once the system has cycled.
Zone 7 A/B
Keep your words sweet. You never know when you may have to eat them.
Richard