Really a crappy position to be in, and unfortunately most of us have been there. Battled ich a few months back after picking up a pair of bangai cardinals. They died within 48 hours and unfortunately infected my 6 line, ocellaris and ywg.
I treated with Sicce Hypercure...but honestly dont think it had any effect.
I fed pellets with garlic and luckily they were strong enough to fight it off and survive.
If getting them out of there and into a QT tank or pail to treat with copper is an option id do it.
I was in the same position as u conflicted wether to run the tank fallow or ride it out I chose to ride it out soaked mysis with selcon and garlic and that really made a difference I lost a few fish but now got a uv sterilizer and still soak the food to ensure the fishes immune system stays strong good luck man parasites suck but we all deal with them try not to stress out the fish to much by putting ur hand in the tank or stuff like that