How are the fish doing @james855
My opinion is a little different, so thought I'd share my views...
The symptoms sound like ich to me. Fine white spots that dropped off after a few days. The life-cycle of ich will mean you may see it come back worse after anything from a week to a month.
I've had it twice. I took all the fish out that survived the first symptoms, used hypo salinity in a QT, did daily water changes, fed them well, and eventually managed to cure several of them. I put hem back in the main tank after leaving it fallow for 10 weeks and within a week they had ich again. I tried again, but the fish all died.
After that I decided to research even more into QT methods, and found a few articles talking about Chloroquine Phosphate (CP). Some huge public aquariums use this with success, so I decided to follow this, along with a freshwater dip with an anti-bacterial medicine, and a worming bath. This combination kills off ich, Brook, Uronema, velvet, fungal and bacterial infections, and worms.
A huge benefit of using CP, is that you can drop some mature liverock in there and feed the tank and have no ammonia or nitrite to worry about!
OK, so what about @Marty
's point about QT for everything else?
Well, I left all my rock, shrimps, snails, nem, etc. In the fallow DT for 9 months. That was mainly due to me wanting to upgrade, and planning that before adding fish again, but I also added new rock for the upgrade, and will be adding more cuc and coral.
For anything new, I will use a small 30L tank, with a couple of kg of LR for filtration, and leave it fallow for 16 weeks. This is almost certain to wipe out ich, Brook, Uronema, etc. Bacterial, fungal and worms, I believe are generally specific to fish, so again, given this period with no fish, should be wiped out.
As for space to put a QT, well it really doesn't need to be big, as it's a temporary holding tank (just like lfs use). Bigger would be nice, but also more expensive for water changes and meds. You could squeeze one under a desk, on a bench in the kitchen or utility room, even in the garage during the summer. Remember, it's temporary whilst you stock up, then you can pack it away and hope not to need it for a long time