There is definitely something foreign in his right gill.
His respiration is also increased, judging by the rate of movement on the gill.
If your CBB shows signs of any deterioration, loss of appetite or lethargy, and you feel is worth it to spend 10 bucks on, try this.
I'd pull him out to a QT tank with the absolute minimal amount of water, just enough so he doesn't feel panicky and get 100% clove oil like this:
https://www.amazon.com/Clove-100-Therapeutic-Grade-Essential/dp/B00PM7S5R6 .This is not the brand I used. I don't remember what brand it was. But just make sure you get one that is 100% clove oil.
An artificial version is tricaine methanesulfonate, but I don't have specific experience with this. It was another suggestion by my vet. I'm assuming there are instructions if you want to use this.
https://pentairaes.com/tricainer-s-topical-anesthetics-ms-222.html
The procedure is a lot less scary than it sounds. I've had to do this with a freshwater Red Arowana dragonfish a long long time ago to try to solve a similar issue. He ended up with some kind of small parasitic worm latched onto the inner side of the gill plate. I'm not claiming this is the actual diagnosis in your case, but this is only for further examination and potential resolution, if you really care about the fish enough. In my case, this was needed on a fish worth thousands of dollars. Yes I was a little anxious, but I needed to do it. The vet was going to charge me much more than what the fish was worth and I didn't have the funds at that time. I was gratuitous that at least he wrote down the following instructions to use at my own risk, which turned out well.
If you are going the clove oil route, put 5 drops, per gallon of water of the qt tank (hence, why minimal amount of water in QT as per above): a drop is sized from a disposable 3 mL pipette like this Wait a while, of about 8 minutes. The CBB's gills will start to slow and he will lose balance and seem to tip over to the side. If not, then drip 2 more drops, per 8 or so minutes until the fish is listless and completely floating around on its side. The gills will still be moving. I'm sorry for the crude estimation, but I can't tell you the exact science of exact dosage of clove oil. All I know is from my vet's instructions and my one time anecdotal experience. This amount of time and drops is safe to use until the fish is sedated.
After tranquilization, see if there is any foreign body on the gill. Make sure you have a forcep or something similar to open the gill flap. Whatever it is, from the video, there appears to be a foreign body in the gill. I am not a vet and this is not professional diagnosis whatsoever. I'm just trying to help.
I know what it feels like to have something go bad with a long time fish and at worst case it would have cost you 10 bucks of clove oil. What have you got to lose, if the your CBB shows signs of futher deterioration and eventual loss? This operation was a success on my Red Arrowana had a long long time ago. Good luck!
