Sounds like he is fiercely defending his territory. My “pair” (not a pair at all I’m just quarantining them together) are starting to chase each other a bit in QT. If you added another, he’d probably beat the snot out of it, based on how he is reacting to his own reflection. Idols are pretty laid back fish but are fiercely aggressive to each other in captivity, more often than not. It is possible to pair them but not worth the risk, IMO. Most who have tried to keep pairs have ended badly. They do OK in large groups in the wild and perhaps giant tanks but that’s not really realistic in 99% of home aquaria.
Powder blue tangs much the same, only aggressive to nearly everything remotely similar in shape, but especially each other. I did think I successfully paired two, only to have them divorce very suddenly and tragically, needing separated. Despite very bad wounds the loser did survive, barely. In large aquaria they too do OK in large groups but small groups end badly, almost always.
Sorry for the tangeant. Do you feed it Nori? Enough that he can eat 2/3 the day grazing upon it? A really fat, full idol is often more laid back. I’ve increased my feeding frequency and the amount of nori I offer and my two are getting along better.
They will be separated as soon as they’re out of quarantine!
@HotRockTarBaby will have one of them, should they both make it. So far they’re thriving!