Looks like ich on the tail fin what I would do is start treating with ick X and aquarium salt 1 table spoon for 5 gallons as ich causes massive electrolyte loss and salt will add back in essential electrolytes do a course of maracyn in food feeding a small amount twice a day for 7 days kanaplex isn't very effective in food as very little of the medication is adsorbed by the intestinal tract in this instance maracyn would be a better option and add an extra air stone during treatment @Stef