Best thing to do is what Reefahholic suggested, Tank Transfer Method. You'll have to find a way to get all of your fish out of your DT and put them into a new uninfected tank. I don't suggest using your current QT because as you said it's already cycled, no reason to contaminate it with ich. A 20 long would be ok, but if you're worried about your fish then try to get two bigger ones (make sure to buy long tanks rather than tall).
Tank transfer method is easy but a pain in the *********, so to lessen the amount of work you want to keep everything as simple as possible. Only things you'll need is a tank, a heater, an air pump, air tubes, air stones, thermometer, pvc pipes for fish to hide in, and of course your saltwater. Having 2 sets of everything helps a lot!
Set your new tank up and make sure that the temp and salinity level match your DT. You should also make sure that the new saltwater has been aerated for at least 24 hours, you wouldn't want to lose any fish to new tank syndrome.
If you're having trouble catching your fish, try draining your DT to where the top of the fish is right below the water line, this works well for me. Try not to use a net, best to buy some type of colander, I used a breeder box with slits in it so the water drains out when transferring which is helpful during TTM. Try your best to do your transfers in the morning. The parasites fall off the fish during dark hours.
If you're worried about ammonia don't be, the fish won't be in there long enough for ammonia to creep up. If you’re still worried you can always buy a Seachem ammonia badge to observe the ammonia levels and buy some Seachem Prime to detoxify the ammonia if it ever creeps up. With TTM, the fish shouldn’t be in the tank longer than 72 hrs so like I said ammonia levels should not be a worry.
If you have 2 sets of everything set up your second tank 24 hours before you have to do your second transfer. Like I said above, match salinity, temp, and make sure your water is aerated.
To do your second transfer, do the same as I mentioned above on catching your fish. Here is where the colander comes into play. You want to try to transfer as little water as possible, so use the colander to drain most of the water out.
Once all fish are out of the first tank, drain the rest of the water so that the tank is empty. Throw away the air stone that you were using but keep the rest. Here’s the pain in the ********* part. You’ll need to sterilize all equipment, except the air pump. Use bleach like Reefahholic suggested, I use 1 part per 20. I let it sit in bleach water for about 30 minutes and then rinse everything off. Once you’ve rinsed everything off you’ll want to let everything air dry. You’ll have to attach the air tube back onto the air pump for it to dry. Once air dried start the process all over for the third transfer.
Your last transfer will be to your QT or whatever tank you’ll use to hold your fish while your display tank is fallow. If you decide to put your fish in a new tank rather than your QT make sure to buy a seachem ammonia badge and some bacteria in a bottle like seachem’s stability. You’ll need to populate the bacteria in your holding tank and bacteria in a bottle will help. If you notice ammonia creeping up do some water changes.
While your DT is fallow make sure to ghost feed.
Take advantage of this Petco sale before it's too late. I hope this helps and your livestock makes it.
P.S. Make sure not to cross contaminate any equipment.
