The triangle shape on the pictures of the fish caught my eye, bit hard to tell what the triangle shape was, is it part of the fishes markings or is it skin that is peeling away.
If it is skin peeling away it will more than likely be Marine Velvet but again the pictures are blurry so it is hard to tell.
If it is part of the markings it will be more than likely Ich is the skin is peeling I would guess Marine Velvet which means you need to act really fast.
Either way a QT with Copper treatment would be the easiest way, for Ich Hypo-Salinity does work but can be a real pain to administer properly as the Salt level drops some of the stats can start to bounce about you will need to maintain Ph which is why my weapon of choice would be a copper treatment, also forget about raising the heat for Ich it doesn't work with Saltwater Ich, heat only works for freshwater
and Saltwater Ich are 2 different beasts with only 2 things in common the first being the name and the second being the whitespots.
If it is Marine Velvet then you can forget about the heat treatment and Hypo-Salinity as both are worthless the only way is to get a Copper based Medicine and treat outside the Main Tank, Marine Velvet looks like Ich except the skin will peel giving a Velvet look hence the name Marine Velvet.
Some Medicines claim to be reef safe but seem to work for some and not for others, a lot of experienced reefers will tell you if it is not copper based then it won't kill Ich, most have some other chemicals mixed in so the fish may look as if it is getting better when infact the Medicines has only eased them, a bit like taking a pain killer it is fine at the start but when it wears off you are back to the start again.
Moving the fish to a QT will stress it out so it can be dangerous but on the same note if moving it to a QT stressed it enough to kill it the chances are it was on its last legs anyway.
Treat all your Fish as it will only be a matter of time before the rest are infected and even if you QT the current fish and get rid of it you will only be putting it back into a tank to get it again.
Set up a QT tank there is a few different ways of doing it, use water from the Main Tank, Mix up some fresh water, you can use RO water but since this is only a temp thing you could use Tap Water but I wouldn't recommend this as your Salt will be designed to be used with RO water, some will say don't use water from the Main Tank as it has Parasites in it but to be honest this isn't really an issue as whatever course of treatment you use the parasite is going to be dieng soon anyway, if the water from the Main Tank has not been kept properly then a Fresh Mix would be the way to go as there is no point in treting the fish in bad water
I like using water from the Main Tank as it means you will be giving the main tank a good water change at the same time, if you have a 15 Gallon QT then you will need to mix up the salt for it, I would rather save the salt for the Main Tank and Nuke the Parasites coming over in the QT, again it's your call both ways aren't the wrong way.
You could do a Freshwater dip this really depends on the state of the fish if it looks pretty bad it may be best to skip this if it doesn't look to bad it may be an idea to consider, you may not also feel comfortable doing a Freshwater Dip either again something to think about.
for the QT you don't need water movement just a Heater and some form of filtration along with somewhere for the fish to hide and sleep at night anything will do I see a lot of QT tanks with plant pots in them, if you have the cash to spare and go with a Copper Treatment then you could sacrifice some of your Live Rock for this.
Also if you go with a Copper Treatment anything in the QT would need cleaned properly there is a few articles online to tell you how to clean them, I used the heater I use to heat up my saltwater mixes in a QT tank before and still use it to heat up my Saltwater mixes and have had no problems anything that can't be cleaned properly goes in the bin or kept for the QT if it is needed again.
Keep the fish in QT for 8 weeks and let the Main Tank sit with nothing in it, there is different views on how long a tank should be kept fishless some say 6 weeks others say 8 weeks, I would just play it safe and go for the 8 weeks after all whats 2 weeks.
Docpoppi said NovAqua for helping with stress, API has brought out a product called Melafix which will do the same job some LFS call it Dope for Fish as they chill out when it is put in the tank, Ive heard stories of folks who put it in their tank anytime they get a new fish just so all the fish are nice and relaxed for a new arrival and the new arrival goes into the tank chilled out.