qt and treat all fishes now.
qt tank is simple barebottom glass box, even tub, with an oldfashioned external airpump and a corner floss filter.
Here is where it gets dicey: feeding your fish will generate ammonia---ammonia will kill your fish: and there are very limited ways to get ammonia out. One is carbon, which sucks it right out but must be changed very often [its sucking ability depletes fast, AND once it fills up it starts releasing everything it collected] like nearly daily when there's little carbon and a lot of problem.
One is water changes---
BUT you can't run carbon with medication or copper: it sucks IT up too and ruins your treatment; and you can't be doing water changes with a treatment, unless you have a reservoir of correct dose treated water to do it with, and that's a pita.
SO...the gentlest and easiest and still effective treatment is hyposalinity. You CAN use carbon with hypo,and you CAN do waterchanges. Hypo WILL kill ich. It is as good as copper for most species.
With tangs, use a 48 hour slow topoff with ro/di water to adjust salinity to 1.009. You will see ph changes. You can adjust this with baking soda. Do not use regular aquarium buffer: it has other things in it.
YOU NEED A REFRACTOMETER AND A PH METER to do this safely, no messing around: this is too delicate.
Mark a 'fill line' on your qt tank to note where water is at perfect salinity re evaporation.
Use one corner filter for every 15 gallons. Change often.
Fish must remain in this tank 4 weeks AFTER the last spot has vanished.
YOU MAY USE PVC PIPE or artificial plants, castles, no problem. Just clean them thoroughly after treatment. Do not use sand, rock, or add inverts. This treatment is lethal to them. Your fish will tolerate it fine.
5 gallons per inch of fish is the proper size of qt tank.
Your main tank should remain fishless for 6-8 weeks, so after your 4th bumpless week, you can start bringing salinity up to match tank very slowly by topping off with salt water.