I'd personally do TTM if it were me but the powder should work as well. Make sure there is no light over it as chloroquine phosphate, the active ingredient in the powder, degrades under UV. Also, any water changes, just put the same amount back into the tank... i.e, remove 10 gals for water change, dose back in enough CP for 10 gals. I'm sure stuff you already know but just wanted to mention just in case.
FYI, freshwater dips mainly provide relief from flukes and marine velvet, but not ich, as the theronts burrow under the skin and are unaffected by the freshwater dip. But if you do the dip and a bunch of white specks fall off, then at least you know you are dealing with velvet, which can also be treated with CP.
Good luck sir!