tks noelfishland. good idea to remove the flagtail to be sure. did a water change and put salt in the tank today. however i noticed that some red are showing up from the bottom of the scales. Anyway, still relieved that its probably not bacterial or fungal infection and hope the water change fixes the problem. The flagtail could be the culprit. In case my RTG survives, can I still put the flagtail back later in the tank ? I see in the net that a lot of people has a flagtail swimming with their aros without any problems.I agree with the first two guys You need to do water changes and the flag tail Is probabley sucking the slime coat of your arow so I would move the flagtail and add extra salt I Hope it gets better but if you do not move the other fish until it has a chance to heal I do not think it will get better,I say this from experience i had my flagtail kill fish in my tanks before the same way