minimum 2-3 weeks after the last sign of any ich on the fish.
I'm assuming there are too many fish in addition to the angel to treat in the 10 gal
hospital tank. The risk you are proposing is much of the DT BB may die off during the treatment, meaning it will take a long time to reestablish the BB colonies after salinity is back to normal. You could therefor be looking at significant daily WC
in the DT for up to 10 weeks (4 weeks treatment total plus possibly 6 weeks to reanimate/reestablish BB). The investment in a new 30 gal
hospital tank requiring less water and for a shorter period of time might be worth it.