Fat Healthy Yellow Tangs Losing Interest in Food

[holdyourlight]

I am having some issues with what appears to be appetite suppression in my yellow tangs, they also seem to be breathing a bit heavier than normal. I bought a trio of them off DD in the spring. QT'd like all of my fish with tank transfer method, and i have a disease free DT. They did great in the DT, (125g 6ft tank) for a couple months, no aggression issues and ate everything. About 2 months ago, one of them just slowly stopped eating. Now it is happening with the other 2. They are very fat, so i have some time, but i can't figure out what is going on.

Parameters are in line as far as the usual stuff. It's a FOWLR. salinity 1.023, temp 78, trates ~10.

I took carbon offline 2 weeks ago to see if that would help, but they still have no interest in pellets or nori, which they used to gobble up, and barely any interest in LRS Frenzy. They still show some interest in frozen mysis, which i only feed a couple times a week. Could carbon lead to appetite suppression? If so i wonder if there could be residual carbon fines in the water.

All the other fish in the tank have healthy appetites. They are not being picked on, nor do they pick on anyone.

I was thinking of picking up a polyfilter to see if it picked anything up.

If anyone has any ideas of something i've overlooked or anything i can try, i'm all ears!

 

[Dmorty217]

What was the last wet thing added to the tank and when?

 

[holdyourlight]

What was the last wet thing added to the tank and when?

The tangs were the last wet thing added to the tank. And that was in May.
There have been no fish added since then and there is nothing else in the tank (besides fish) other than rock or sand. No coral, no inverts, nothing. The only things that have ever been added to this tank, which was started a year ago are: Fully QT'd fish, dry rock, dry sand, and water.

I don't have a pH probe any longer. I was thinking it could be an oxygen issue possibly after reading that tangs are particularly sensitive. I opened up the windows closest to the tank today as temps are in the high 70's as a sort of hail mary...

 

[holdyourlight]

Still completely puzzled by this. The situation has not improved in the last couple weeks. I dosed PraziPro today even though i am 100% sure there are no flukes, and i am running out of ideas and have tried everything i can think of.

I did see a couple of black specs on tangs from time to time, so i guess i could have paravortex (black ich), but it just as easily could have been some sand...