I've been using tap water in my sps dominant tank for 15 years which has a heavy bioload of fish. Nitrates average 15ppm and Phosphate around .1 with the use of vodka and lanthanum. I get that people believe rodi is the only way but I do question how many of them actually have tried a window and compared results.
Tim helped me set up my Apex and we had many laughs over his bad water. I think we have shared killing many acros together. He was one friend I'll remember in my life and I was blessed to have met such a genuine soul. God bless you my friend.
that tang just looks like he's stressed bc of his lighter colors, all you have in that tank is a pvc pipe and nothing for him to hide in. If you wanted to treat him then all you really need is to do hypo salinity and keep the water clean. Feed him whatever he's used to eating to keep him strong. I would put thought in what in your display tank is causing him to stress in the first place.