Hospital tank set up advice, please comment on plan

[Delphinus]

Well, the time has come that I have to admit that I need to treat some fish I have. Quick summary: in January I moved most of the fish out of my 115g cube into my new tank. I left behind two butterfly fish. The stress of the chasing of fish seemed to give the butterflies themselves a case of ich. 3-4 months later now, it is clear that it is not going to go away and it's bad enough that I think they need to be specifically treated.

It's very frustrating. They eat like pigs, which must be a good sign, but the spots are just getting worse every week.

So .. I have a 20g tank that I have kept for just this scenario. I've just never had cause to actually set up a hospital tank (I did once try on some newly purchased fish, I tried to QT them, then I tried to hyposalinity them but they all died. So I've been very gun shy to try this again.)

Anyhow.

I also have an Aquaclear 300. I will put the Aquaclear with sponge on the 115g tank tonight and let it run to cycle the sponge.

How long should I let it seed on the main tank? 24 hours? Or longer?

When the Aquaclear is seeded well enough, I will fill the 20g with 100% tank water from the main tank and move the Aquaclear over.

Once I begin treatment (and I haven't quite decided if I should do hyposalinity or Cupramine, any thoughts on this?), do I need to do daily water changes to keep the ammonia in check?

I don't have an ammonia test kit. Should I get one? Or just do daily water changes?

How big of water changes once treatment begins? 100%? 50%? 25%? Every day? Every 2nd day?

Any advice appreciated, thank you!