I kept a CBB for two years succesfully. It ate out of my hands everyday! It died when I had two extremely agressive fish that got too big and basically bullied the CBB out of the way of food. CBBs generally give up eating pretty quickly if there's too agressive fish. I removed the agressive fish, but not in time to save the CBB.
One of the few fish I actually mourned. He was like a puppy. Came up to me everyday and would munch partially thawed frozen food from my fingers. I miss him.
That said, a couple ideas, I don't know if you can save it or not. These fish are extremely sensitive to almost anything! Lol, hense expert care required.
1. Yes, Any rocks with aiptasia and/or tiny worms Please add!! I got a CBB because my tank was covered in Aiptasia. I was to the point of tearing down my 340 gallon tank. The CBB devoured every aiptasia in my tank. I can't begin to tell you how many tens of thousands of aiptasia I had. My CBB devoured them all in probably 6 months. To this day, I have no aiptasia thanks to my awesome CBB. You don't realize how much these fish eat, until tens of thousands of aiptasia were gone in 6 months! (I'm not lieing on the number....) I had aiptasia covering my sand bed. I couldn't see my sand bed anymore. Over 100 pounds of rock in my tank were covered in aiptasia big and small. I had aiptasia starting to grow on the glass there wasn't room for them to spread anywhere else.
2. I can see what the fish is doing in your video. A lot of people miss this. But, He's chasing his reflection. I see his reflection in the video and he's going back and forth because he sees another of himself. Either reduce the reflective nature of the side glass or let him settle in for a couple days. This is most likely very normal acclimation behavior. He wants to get to the other fish and he's smartly swimming to the "edge" of the obstacle (the glass) to try to get to the other fish (which is the corner). Only he can't find a way around it. See if after a few days he gives up. I have wrasses and tangs that do this everyday, check to see if their reflection is still there, but they usually get distracted by something else then come back later.
3. CBBs are very very timid eaters! To the point they're almost scared of food in the water column. A few things to try (I didn't do this, and if you want decorative clams I wouldn't do this), but try putting food on a clam shell, or feed raw clam directly on a clam shell on a rock somewhere. Put it in the same spot, same time everyday, remove within half hour (or sooner) and see if he eventually finds it. You could do this with Matchstick food as well. I used Match stick and just smooshed in on the glass in the same place everyday. He came and pecked at it constantly. The idea is establish a routine of at least once a day putting out a "dog dish" in the same spot.
4. Lastly, try just holding frozen food near him with your fingers. (Mysis is the easiest, but others that fit in his beak might work). Hold it in the water near him, the same place, the same times every day. You never know he might surprise you and start eating out of your hand. After a few times of this, if he eats, he will find your fingers very safe to eat from. Mine bit my fingers a couple times trying to get food off them. Lol. He should eventually search for pods as well throughout the day, but that takes time. Supplemental feedings to get him feeling safe and comfortable with routine is key initially.
Trying some or all of those things, may not be enough to save him. He may just have given up on eating. It's hard to know for sure. Try things, but there's no guarantees!
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Hoping for the best.
