So unfortunately, I don't have a good update

I noticed the larger tilefish started getting somewhat lethargic over the last two days, which I attributed to a possible stress from hypo, but today he barely ate and seemed to get much worse over the last 12 hours.
He's having a really hard time swimming normally. One minute he'll swim fine around the tank for a bit, then the next he looks really tired and starts swimming tail up and tilting. It's like he doesn't have enough energy.
I also noticed the smaller tilefish started nipping at him again. I've been watching them them a hawk and replaying the camera footage every day and I never saw any bullying until today. I wonder if perhaps something happened last night, because today the larger tilefish suddenly looked much worse. The smaller one is still very active, eating, breathing normally etc., so I don't think it's a water quality issue (ammonia 0.5, pH 7.8, salinity 1.010-11).
I separated them again, but I don't know if he's going to make it overnight. I feel like the smaller tilefish might've been slowly wearing him down behind the scenes or at night, where even the cameras couldn't see. Yesterday he was still eating, although a bit slower, and today he's just looking awful.
I feel really guilty for removing the barrier. I never saw any signs until today. They were always both out and eating happily together. I guess the lesson is that even fish as peaceful as tilefish can bully each other in ways that are really hard to notice.
This is from today, first video shows the nipping, second video shows the larger tilefish swimming with his tail up.