You should have assumed that all new additions of fish has the ich. Automatic assumption.
Now you have to catch ALL the fish out for quarantine for at least six weeks to eradicate the ich organism. During this period, you actively treat the fish and allow the invertebrates to be fishless.
Problem. Big one.
If you have nitrification filter that you can quickly arrange it would be a little better. Guess what, a person who would just put an unquarantined fish into an established and stocked tank typically would not have prepared another nitrification filter in advance.
If you have non-live rock as the filter medium in DT or if the live rock has not developed any invertebrate growth on it (which dies and pollutes the QT with ammonia), you may split the medium: a part to support the invertebrates the other the fish.
The use of the diatom filter (such as vortex D-1 or DL-1) can likely slow down the spread of ich, like the dilution effection of the ocean if infestation is now still low. This will give you time to cycle for enough nitrification capacity to support the fish in a QT.
Or you can do frequently water change (may be daily) during the six-eight weeks of separation of fish and invertebrates. Or you may try Amquel or the likes to precipitate the ammonia in QT.
May be garlic can reduce the spread of ich, some say.
These are the options.
