You’ve got a PBT in a 4 footer right? That’s not the right environment or ideal conditions, and if you have a purple and other fish, O2 level is definitely not going to be ideal for the PBT short of pumping oxygen into the tank (no one does this) or hooking your skimmer up to a MASSIVE CO2 reactor.
That’s the point. I don’t mean to offend but you’re not providing the PBT with ideal conditions, the fact it isn’t using all 4 feet is irrelevant. That’s like jumping out of a plane without a parachute because someone somewhere survived the landing. You’re essentially using the anecdote of a PBT (I’m assuming you’ve added fairly recently) in one instance. For that one instance, we have countless instances of PBTs who don’t eat and whither, die due to stress, cause an ich/velvet outbreak or if they survive, kill everything else in the tank.
A quick google search will show you the rule to your temporary exception, that’s my whole point.
I’ve had acanthurus tangs orange shoulder, powder blue, white cheek) in my 260 gal and I’ve had a purple and desjardini, and the acanthurus scour the whole 7x2.5 area, the only time they didn’t is when a bigger meaner fish (my first blue face angel) would harass them all out of a 3 foot block on the 7 foot tank when they were first introduced.
experienced reefers know this, which is why when we keep tangs in tanks that are too small, we have exit strategies, what is your exit strategy? I’ve no idea how much a powder blue/brown costs in the UK but I’m assuming it’s more than here in Aus, so you’ve basically got an expensive time bombs you’re dealing with statistically speaking.
I’m not saying that you’re doing wrong by the fish (I’m not a tang cop, I too am in the works of keeping a bristletooth in a 70 gal) what I’m saying is that you’re essentially asking a loaded question (which is a fair one BTW). I also answered that question too, which is no, people do not over exaggerate the requirements to care for these fish long term, if anything tank size is underestimated as most reef based acanthurids scour reefs in large groups to feed. They aren’t like pomacanthids, or anemone fish where once established they lurk in a set territory or anthias where they sit against the current in the water column above a reef to feed.
in absence of providing the ideal environment, we need to do the best we can to make sure we meets the animal’s needs. I’ll put it to you this way, you’re not going to leave a border collie or a cattle dog in an apartment 24/7 right? If you do you’re guaranteed to end up with a neurotic dog who wrecks your apartment. Which is why you try and give the dog all that you can to make sure you’re meeting its needs, fish should be no different.