id also go with the nutrient deficiency. i took fish and wildlife technician in college and we did studies on fish in different rivers/lakes. the only time i saw gill defects was either close to farm land where pesticides have been used or in the winter time when food is limited. the rainbow trout we caught had it so bad that there was no gill cover left all you could see was the bright red gill filaments. the trout had sunken stomachs from having nothing to eat. and the fry that we found all had it on both sides. the larger ones had it one side much worse then the other.
i feel your pain when it comes to switching the food your fish take. my 3 gatf r an absolute headache right now. i feel bad starving them but ive finally got the middle sized one to constantly eat carnistix. the big one hasnt eaten for at least 2 weeks. but he has just recently started biting and spitting out the pellets which i figure is a start. the smallest one hasnt eaten but hes still fairly new. today i thought he was going to try one but turned away last second. id highly recommend the garlic because it stimulates their hunger. i always add a bit of garlic to my beef heart/shrimp mix that i give to my brackish tank.
Well luckily he still eats and swims around normaly, so I dont think parasites are an issue(never been fed live foods either), I added a half does of prazi pro tonight as a preventive and bumped the temp up to 83. All I can do now is wait and try to get'em on pellets, or at least stuffed foods.