Originally Posted by
jeep
It loiks like infection or fungus. This can be caused by prolonged poor water quality, possibly as a secondary to flukes. Your water change routine seems decent enough but there can be more to water quality than just water changes. Bio-load plays a big role. As for treatment, I've used Kanamycin + salt, others recommend PP. Either treatment should be done in a hospital tank. Also increase your water changes and try to get nitrates to <5. Hopefully someone with a little more experience with gill issues can weigh in.
I would get a jump on this pretty quickly. Gill damage of this nature is rarely cured 100%.