when I turn them on after 5 hours, they seem to dissappear.
Hmm. I wonder if, as Michel says, they are simply less visible when the lights go out. Again, ich won't appear as disappear in a few hours span. But I guess if one couldn't see them because of the lighting, it would seem that they are disappearing...
I would get that Tang out of that tank and into a quarantine tank as soon as possible. I am still hesitant to offer suggestions on how to rid the ich from the fish as I am not sure it has it. Although, you did say that your other fish just recovered from ich so it is definitely possible.
There are only two proven remedies for ich. Hypo salinity and copper. I would recommend hypo before copper, as you will have to monitor the copper twice a day to make sure you have a high enough dose to kill ich but not too high so that you kill your fish. If you do decide to go the copper route, make sure you keep copper levels under .25 ppm
and over .15 ppm
. Leave the fish in for four weeks.
If you go the hypo route, adjust your quarantine tank to a SG
of 1.009 and leave the tang in there for 4-6 weeks.
, you cannot use either of these methods in your display tank if you keep or ever plan to keep inverts. A QT tank is necessary.
Perhaps someone else will chime in with some other suggestions.