Brooklynella and ich??

So far around the past month or so my Powder blue tang has had ich, but it had not seemed to affect anyone else or caused any loss of color, appetite, or attitude(didn't hide).

Around 2 weeks ago i managed to find 2 semi Picasso clowns for a decent price 60 bucks apiece. Although the store wasn't lets say the best quality. I brought these things home and put them in quarantine for 2 weeks, showed no signs of anything so I acclimated them to the dt.

I came home a day ago to find one of the clowns on the floor. Ive never had a fish do this ever before. None of my other clowns ever jump or anything.

And then today about 48 hours later i notice the powder blue tang appears to have ich and some sort of white cobweb looking like stuff hanging off him.

I cant get a picture in focus enough to show but after seeing pics of other fish with it. I'm thinking the new clowns may have brought it in with them.

Now my question is the tang is not gasping or losing appetite, but is starting to show some lesions, and scratching himself on the other yellow tang who has none of this.
I also have six domino damsels that are starting to get ich now, possibly too many bacterias in tank. they like to now swim in the power head and show some loss in color. Now I only have one qt tank, even though the fish have what i think is two separate diseases can i treat that at same time?
like be doing the copper treatment in qt tank and pulling tang and clowns out for the formaldehyde bath and then return them to qt tank, is this safe?

also
temp 80 degrees
ph 8.3
alk - 7
salinty 1.024 a lil high im adding r/o water to bring this down.
nitrite - 0
ammonia 0
nitrate - approx 5
calcium 400 ppm
125 gallon tank with with fish, corals, and etc.
lighting 2 x 250 w mh w 2 65watt actinic and 6 led
4 power heads
200 lbs of live Fiji rock
refugium w protein skimmer underneath

 

It sounds very frustrating and that you are trying to do everything right. Hope this helps at least a little. GOOD JOB ON HAVING A QT SET UP.