Gill flukes, velvet and even low oxygen are general reasons this may happen as gills will be affected by the above listed. With flukes, you will see fish red or swollen with rapid breathing Neither, fish acting lethargic I have noticed this or swimming near the water surface None, hiding in the corner of tank or behind rocks Neither, loss of appetite Yes, then they get skinny and die, shaking its head none, flashing/darting none, develop clamped fins I assume this is similar to how an asian fan closes up when not in use? If so, then no, , or scratching against objects every now and then, but usually scratch against the bottom of the tank (bare bottom). They may also exhibit what looks like yawning from gill irritation develop None, cloudy eyes 1 scribble has pop eye like symptoms and bubble in the eye and loss of color prior to loss Most have been hard to tell, but the atlantic blue tang did lose color half of its body then perished. with low oxygen don't see them gasping for air, gasping at surface, loss of appetite yes, lethargic yes, hiding and labored breathing no.
With velvet, fish will scratch body against hard objects, lethargic behavior, Loss of appetite and weight loss, Rapid, labored breathing, Fins clamped against the body, and typically stay at the surface of the water, or remain in a position where a steady flow of water is present in the aquarium. Base on the conversations with you and Jay before, I don't see any signs of velvet.
