WaffleWaffle22 & pnavarro: Aside from inaccurate observations regarding the filter/water current, your responses are uncalled for. tref2010 is here asking for help, instead of flaming him, why not recognize that he is trying to do his best. There is not a single one of us in the hobby who has not inadvertently brought an animal to its untimely demise due to a mistake or lack of knowledge. It is an unfortunate part of the hobby that can be minimized through help and advice from more expereinced aquarists. IF this is to much for you, then simply do not reply to the thread, your negativity is neither productive nor wanted. Please keep this in mind. Thank you.
Tref2010, please ignore the nay-sayers, I am glad you are asking for help and I will do anything I can for you. From what I can see in the video it looks like your tank has a sand substrate, HOB filter and pre-set heater, is this correct?
In regards to the decor, while I personally do not find the plastic plant and bridge attractive, my opinion nor that of any one elses matters. They are not harmful to your fish, and in fact both of those objects may well serve as a perfect territory/host for your clownfish. Furthermore, the filtration you have is adequate, especially with the yellow tail damsel and juvie CB Percula clown. I often times keep BREEDING pairs of clownfish in a 5g tank with nothing more than a sponge filter and flower pot to host in, I even have a few pairs hosting in Sierra Nevada Pale Ale Bottles (One of my favorite brews..).
Now, to get back to the problem at hand. You say you got your Percula from Petco, while this in itslef is not a crime it can be risky. Of course, buying any fish from anywhere is risky. However, my understanding is that Petco employees cannot use copper etc. in their systems due to corporate policy, but this is not a place for that debate. So, it is very possible that the fish was exposed to a host of pathogens at the store, and it may just now be showing. It is also possible that the Percula picked something up from the damsel in there. Impossible to say.
However, since you have said the fish was eating and otherwise seemed healthy until this morning I think we can rule out ammonia/pH poisoning. This typically occurs in transit and while it may not fully manifest for up to a week, it is unlikely that transit stress is the problem. At this point I feel fairly confident in diagnosing the fish with a bacterial infection, or possibly a secondary bacterial infection induced by a protozoa/parasite and or stress.
While many of the symptoms I can see would indicate Brookynella, I am not sure that it is in fact Brook.
I need a few more answers to help as much as I can, so please answer the following questions:
1. It appears that there is some undisolved salt on the rock the fish is hovering above in the picture, is that what I am seeing? If so, did you add salt directly to the tank?
2. What and how often have you been feeding the fish?
3. When did you get the damsel?
4. Do you have any pics/vids of the fish either from when you first got it or from the last few days, before it began displaying the illness?
5. What is your water change schedule.
Answer all these questions the best you can, and provide any other info you can and I am sure we can get it ironed out. And we will get you on the right track for success.