First, do you have a QT tank that the tangs can be moved to?Well, since you said nothing is visible to you that should rule out crypto which is often referenced when talking about tangs..but you should be seeing some white spots. I googled looking for diseases/parasites that Tangs are PRONE to...and from Bluezooaquatics I did read the following:"[There are several other protozoan infestations of which the marine aquarist should be aware. They are ]Amyloodinium ocellatum[(sometimes called marine velvet disease) and two additional ciliate protozoans, ]Brooklynella hostiles[ (sometimes called anemonefish disease) and ]Uronema marinum[ (often called uronema). All three of these can affect tangs, and all can be devastating to your system if not treated quickly. All three infestations present in roughly the same way (skin damage, rapid breathing, rubbing on rocks and substrate, and extreme lethargy) and are the results of very similar parasitic organisms with almost identical life cycles."]It was the 'lethargy' part that caught my eye. I hope someone more knowledgeable and experienced than me will respond. Here is the link to the article I was reading (um..hope it's ok to post things like that..I don't know all the rules yet) https://www.bluezooaquatics.com/resources.asp?show=378
