Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Since it is not identified, yet, I would start with some extra water changes, and rinse out the filter media. The less debris in the system, the better. This will remove microorganisms that drift in the water, so there are fewer to re-infect the fish, and remove organic matter that any medication might stick to, so any medications can target the injured area.
If there are no Anabantoids in the tank I would add Melafix and Pimafix to the tank. These are good on external issues. These do not enter the fish's body to attack internal problems, though. If it does not start looking better pretty fast (few days) then switch to stronger meds.
I think you are asking about the red area at the bottom of the operculum and perhaps red abdomen? I have seen some Phantoms with a blush of red in this area. Is this color new?
Hard to see if either area is swollen.
The red area of the operculum could be an injury. Often, in chasing and being chased the fish may bump into something, or else another fish may nip. This is an odd area for a nip, though. If it is an injury, then clean water may be all that is needed to help the fish heal.
The red abdomen looks more like something internal, just under the skin, such as bacterial infection. (If it is not the normal color of the fish). If this is part of the problem, then internal antibiotics may be needed to fix it.