Is this the same 20 gallon tank you were having Nitrate problems with a few weeks ago?? If so, tangs are highly susceptible to ich when stressed, and it's going to be extremely stressed in a tiny 20 gallon tank, let alone with the high nitrates...... those guys belong in a much, MUCH larger tank (Minimum 180 gallon!!!).
The meds you have are likely copper based, which cannot be used in your tank. Once in there, they cannot be gotten rid of, as they permeate everything, including the silicone seals, and that tank can never be used to house inverts or corals again, hence the need for a fish-only QT tank.
Without a QT, you can try hypo-salinity, but you will need to rehome everything in your tank before doing this..... neither the shrimp, anemone, or anything else will survive.