Update: Three of my Pajama died, that makes 4 fish including the clown only today :worried. I also have among others a large Naso Vlamingii he is my prized possession as I play with him and have a really special connection with the guy, he is 14-15" long so he is a pretty big fish and tame as my dogs

He is starting to breath rapidly and starting to develop a red line at the bottom of the fin on the back + a red sore behind his right gill where the pectoral fin starts(I think it's called pectoral fin). I am sorry if my English is sub par but Vikings don't speak English as their native tongue

I do not want to loose this guy I have had him for years and love him, please advice. I don't want to loose any fish! I am at the verge of treating the whole tank with Metronidazole as there was a guy up here in the north who told me that would end all my problems. Except he said it could kill all beneficial bacteria too, so I had to stock up with a lot of Seachem ammonia stop and an alarm? But.....I don't want to do that unless you seasoned guys could ease my mind a little regarding that and say go or not go.
Please guys advice, I am getting desperate here I've had most of these fish for almost 10 years and 7 of them are prized specimens of different tangs plus plus. I re-dosed Hydroxychloroquin sulph today with 5mg/l and put in lot of air stones in the sump plus a lot of other measures to increase O2 which I hope will ease their breathing. I have also lowered the salinity down to 1.018 for the time being and plan to lower it even more tomorrow.
Am I at the right track?
Sorry if my post is a little messed up but I am really messed up too, I feel so helpless seeing the fish struggle like this. It really BREAKS my heart.
This is painful learning, for ten years I haven't seen any disease in my DT then I did the most stupid move of all.....Introduced a Moorish Idol which had lived in a buddy's tank for two years with no problems. I thought that was quarantine enough but obviously I was painfully wrong. I hate myself for this move. Maybe I should go back to Malawi Cichlids and admit defeat hehe