When you test if your water parameters are fine then it looks more like an injury you would have to monitor very closely just in case it develops a secondary bacterial or fungal infection you could add some Indian almond leaves as they have antibacterial and antifungal properties you might have to add one leaf per gallon to get a beneficial effect if you notice it get worse then I would do a course of kanaplex or maracyn2 Which ever one is more readily available @EggShappedFish