I think you will find it is a combination of a dramatically increased bioload and feedings. Keep up with the water changes, ensure any filtration equipment is cleaned regularly in SW
and feed very sparingly. Some nori for the tang to graze on and light feedings every second of third day should be fine for now. If the QT
was cycled prior to the new additions, it should allow the biofilter to catch up soon enough.
Hopefully the main tank will be ready soon...