Bangaii Suddenly Aggressive to Pajama Cardinal

[EricR]

My Bangaii cardinal has gotten aggressive towards my pajama cardinal just within the last 5 days or so,,, very often and relentless.
They've been together in my tank for 16 months and had always gotten along peacefully until now.

My questions for the more experienced are:
1. How long would you let this new behavior play out to see if it just resolves itself? *no significant injuries (yet)
2. How likely is it that sticking the Bangaii in an acclimation box would actually help? *feels like a temporary bandaid to me but maybe not
3. If acclimation box for Bangaii makes sense, how long do you guys usually go?

Video:
*hard to get a good one -- the minute I pull out my phone, behavior changes,,, but this is pretty much what I'm talking about



They used to just float around the middle of the tank together and even seemed to cooperatively hunt pods off rocks in the evening together.
Now, the Bangaii mostly forces the PJ into the right corner of the tank and kind of just corrals it there, occasionally nipping.
I see some minor bite marks on tips of fins but nothing major yet so would rather let them work it out but maybe that's a bad idea.

Basics:
-- 40 gallon breeder tank running since 10/2021 with mainly soft corals
-- Only 4 fish; solo clown, tailspot blenny, Bangaii cardinal, pajama cardinal
-- Pajama was last fish added to the tank about 16 months ago,,, Bangaii was about 1 month before that

 

[Uncle99]

Me, I’d do nothing at the moment if no damage is being done. Always best if they can work it out on their own.

As long as they both eat, and the harassment is not sustained, just observe.

They tolerated each other for 16 months.

Do they settle at night?

 

[EricR]

Me, I’d do nothing at the moment if no damage is being done. Always best if they can work it out on their own.

As long as they both eat, and the harassment is not sustained, just observe.

They tolerated each other for 16 months.

Do they settle at night?

At night, they seem to just kind of go their own separate ways, as far as I can tell.
Just, during the day, it's like constant harassment now.
All fish are aggressive eaters at feeding time but the PJ is now more docile.
Still eats though, just doesn't jump into the feeding frenzy mix with the other fish now.
*I see some notches out of fins but not what I'd consider major damage, although I'm no expert

 

[Uncle99]

Many keep these two fish together without issue.
16 months then that…..wow….but I guess anything is possible in the box.

Maybe try a mirror to see what that does to the situation, maybe some new rock for new territory, I can’t see how separation is going to buy much.

 

[EricR]

Many keep these two fish together without issue.
16 months then that…..wow….but I guess anything is possible in the box.

Maybe try a mirror to see what that does to the situation, maybe some new rock for new territory, I can’t see how separation is going to buy much.

Silly question -- does the mirror have to be right up against the glass (touching) or can it be an inch or two away.
*I've heard about the mirror trick before but never tried it,,, not sure about the water --> glass --> air --> mirror interface in terms of refraction, if that even matters