He may have some sort of infection going on, not eating is definitely not a good sign, and keeping him in a biorb wouldn't let him recover properly because of the poor filtration system and small surface area. It is doable though, I kept one of my moors in a biorb for a few months without problems!
But, if you only could, sell the biorb and get a nice 100 liter tank for the moor instead
If you have enough filtration on that (1000liters/hour minimum) he could even have a little buddy there
Preferably another moor.
How big is the quarantine tank and what kind of filtration do you have?