It is velvet, trying to get out of display tank into qt

I doubt it. Your problem started weeks ago. If it was marine velvet all your fish would be dead my now. It's way too long to be marine velvet.

Unless you ID it under a microscope you will not really know what it is. I would do this on the first dead fish, that would probably save the rest if the right treatment is applied.

For exemple, if you have brooklynella in your tank, copper won't have any effet on it and you must use formaline. Brooklynella is a good fish killer as well. This is why it is so important to properly ID the parasite to begin with. Loosing time with the wrong treatment is a sure way to kill your fish.

after 15 years in the hobby, my experience has been once a tank has had ich, it seems to hang around. Most fish will get sick, the strong ones will build up a tolerance or perhaps somewhat of an immunity to it. But every time I add a new fish to my Marine tank, it gets ich, none of the other fish do however.

It seems, from my experience that healthy fish will survive it, but the parasite never seems to go away in my saltwater tank.

I have not had an outbreak in my freshwater tank for years, but then again, I have not added any new fish in years! I would agree though with other comments, to keep the fish healthy I would feed more often and perhaps, a larger amount than normal (as long as they are all eating it). That way it keeps their strength up, there could be a fish or two that will stop eating during the nastiest part of this parasites life cycle those are the ones that are hit and miss. If they were eating well and generally healthy before, they should survive it.

I am done with adding copper of any form, it didn't work for me and all it did was make two of my fish blind and then died weeks after. I just let the cycle run it's coarse and since all my current fish have had it before, I think they are fairly hardy.

AND I just noticed this was a dormant thread...not sure why I responded to it months after..sorry about that.

I know this is an old thread - but... it's a good point to revisit occasionally. It is entirely possible to keep ich out of a tank. My tank has been ich free since November of 2012. No outbreaks, no spots, no flashing, no scratching, no nothing. In that time I've added fish, removed fish, accidentally dropped my salinity down to brackish levels twice, and wrapped the tank in plastic and subjected it to a 2 month home renovation that required 3 weeks of jack-hammering (with an actual jack hammer) out a tile floor on three sides of the tank and poisoned half my corals with self levelling cement dust. If there was any credibility to the notion that it's 'always' present and only flares up when a fish is stressed, my powder blue tang should have looked like I'd dipped him in salt at least 3 times that I can think of.

I've messed up just about everything with this tank at some point or another, and the longer I go with this hobby the more I realize I don't know very much about anything, but what I *do* know is that we need to retire forever the idea that ich is always present. It is not. Not worth the trouble to get it out once in... maybe. But possible to eliminate and keep out forever? Definitely. IMO keeping it out is probably the most straightforward part of keeping a reef tank long term, if it's something someone chooses to do.