Welcome to Reef Central. Sorry your fish is having a problem.
As far as I can tell, the fish is not breathing above normal rates. Just to be sure, couunt the number of 'breaths' your fish takes in 15 seconds and multiply that by 4. If it is over 120, the fish is having breathing problems. But, like I wrote, I think the fish doesn't seem to have labored breathing.
Thus, I tend to put the possibility of gill flukes, on the back burner. This leaves a bacterial infection. The redness on this fish in the gill area is something I've inspected under the microscope. More times than not, it is a Mycobateria infection. One of the worst is
Mycobacterium marinum. If it is a Mycobacteria infection then there is no treatment but
beware: Humans can give fish
Mycobacterium marinum and fish can infect humans with this bacteria. It transmits
vis a vis putting hands into the aquarium water.
Do you have access to a microscope? You would take a gill clip and inspect it under the microscope and look for this bacteria.
Hopefully it is not
Mycobacterium marinum. It may be another bacteria or the lesser chance of it being gill flukes. (With gill flukes, the breathing should be faster).
Having written all this: Treat the fish with an antibiotic. Most fish bacteria are Gram Negative bacteria, but this infection could be either one. Treat with both Maracyn 1 and Maracyn 2 (at the same time).
I don't know what you have in your fish medicine cabinet, but it should contain the Basics as listed in this
Medicine Cabinet post. So if you have Kanaplex, use it now. Nitrofuracin Green (NFG) would be a good choice, too. So, there are options.
Just to be conservative. . .Treat with Praziquantel (for flukes). Praziquantel can be combined with antibiotics Maracyn 1 & 2 and Kanaplex. You can also treat the fish with General Cure plus an antibiotic.
Using Praziquantel
General Cure (which contains in part, Praziquantel) and PraziPro (containing mostly Praziquantel) and Praziquantel powder is an anthelmintic (deworming) medication. However, this medication binds with oxygen in the water to some extent. Thus, when using any of these products it is
mandatory to add an airstone into the QT. A rule of thumb would be to add one airstone for every 10 gallons of QT water, or if the QT contains multiple fishes, add one airstone for every 3 fishes. Run a vigorous amount of air through the stone(s).
Understand if this is Mycobacteria, this fish won't recover after using the above treatments. All the fish in the QT will need to be euthanized. The entire QT system must be sterilized. I hope this isn't Mycobacteria.
