Sorry, you lost some good fish. I think, judging from the photo of the bannerfish (butterfly) that your fish probably were in big trouble, even before you moved them to QT. I'm not good at telling parasites from just pics, but some educated guesses: 1.) I think there's a good chance that your tank has marine velvet. this is a very fast spreading, very contagious deadly parasite. it may be a very advanced case of ich---I just can't tell from the pics. 2.) It looks like you waited way too long to treat these fish; that butterfly did't get that bad overnight. 3.) Anyone who tells you not to worry, when a fish is covered head-to- tail with parasites and a slimy coat is really missing it. Please don't listen to such advice in the future. Check the stickys in the disease section for advice that you can trust. 3.) The cottony patches are probably a fungal infection, usually secondary to what the parasites are doing. Fungus takes a while to develop, another reason I feel treatment was delayed too long. 4.) You were right to get the fish out of your DT, even if you lose them all. You have to let the tank go fishless anyhow and dead, large fish can create a tremendous amount of ammonia; endangering everything else in your tank. 5.) Is this the same tank you were setting up just a couple months ago? If so, its WAY to early to have so many large fish in it. this could easily be the foundation of all your problems. 6.) Here's a cut & paste of what I do to cycle a QT; like Evsalty said, QT is always instantly available.
"In regards to Qt cycling; I've done this for years. Get a HOB filter; I really like Aqua-Clear, they have a big sponge and last forever. Don't use the carbon or ceramic noodles that come with the filter. Also, have some extra sponges on hand, they're cheap. Keep a sponge in the flow somewhere in your DT. When you need a QT or HT, just use the sponge that has been in your main system in your QT filter---the QT will be instantly cycled. When done, toss the sponge and keep a new one ready in your main system.
BTW, Cupramine copper, used in a QT,will not destroy a bio-filter. Do not use ammonia neutralizing products with Cupramine." 7.) Please use this experience to help others when they are told that parasite infestations will "just go away on their own". 8.) While everyone is right on mixing copper & ammonia-neutralizers, I don't think this is what killed your fish. IMO, a very heavy concentration of a parasite did.
