I like to have a small hospital tank. It saves on meds as well as only treating the affected. Some like to treat their tanks as if the whole tank is infected. I chose to do the hospital thing and it has worked.
Look up gill flukes first. Some parrots get gill curl, which exposes red veins in their gill.
Id treat the whole tank because the flukes are still going to live in the water and are probably on the other fish as well. Flukes are VERY bad. Treat for external parasites asap. Its probably an early stage, but flukes can lead to death realllll quick. They dont just attack 1 fish, they are all in the filter other fish and in the water