I know this is dispiriting. I would read up on velvet in the stickies and plan next steps based on that. You should assume that all of your fish are infected now and the parasite is in the DT. Some fish may survive, but that is not to say it won't flare up again. I'll tell you what I did. Warning: it is a heck of a lot of work.
Remove all fish to one or more hospital tanks and treat with copper or CP, and leave fish out of DT for the applicable fallow period (many people go ahead and do the longer 76 day period for ich, just to be safe). I would expect some deaths. Also, move some bio media from your DT to the hospital tanks to seed the bio filter and help with ammonia. Does not matter if the parasite is in the media, because you are going to firebomb it with copper or CP. Make sure you have lots of powerheads for oxygen exchange, and check ammonia regularly, and change water more frequently than for a DT (have to carefully add back copper or CP). Your copper or CP period (30 days) will be done before your fallow period is. So just watch fish, make sure no more infections. Then add back at the end of the fallow period. It is a HUGE PAIN. And personally, I would QT all new fish in copper going forward. Unless you do the QT, there's no way to be sure.
Good luck!