I always use hyposalinity for treatment of multiple fish. It is for many reasons easier on the fish than copper. TTM is also a good choice but likely less doable in your case.
Hyposalinity is quite easy. The biggest issue is to get the salinity right at the start - maintaining it is then fairly easy if you just keep the water level (best mark it with a sharpie or a piece of tape) constant by refilling RO water. Evaporation has a much lesser influence at low salinity than at normal salinity. To minimize even that I usually cover my treatment tanks with a clear plastic or glass lid.
You should also add some 2 part Alkalinity and Calcium solution (go with the daily base dosage) to prevent the pH and alkalinity drifting too low. It would be good to check pH and Alkalinity daily or at least every other day.
I set up the treatment tank clean and adjust the salinity (1.008 - 1.009) and temperature (~25°C). A day later I add the fish (drip-acclimatization over 30 min to an hour). While the salinity is easily maintained I would still recommend to check it every morning and evening.
You will need a refractometer (you should have one anyway) and calibrate it with RO water to 1.000 kg/L (0 "°)
When setting up the treatment tank I would also add some clean "decoration" (clean = no ich on them) for the fish to hide and some bacterial filter starter.
I found ceramic flowerpots to be excellent QT and HT decoration as they are cheap. You can leave them whole or break them to build caves and hiding spaces for the fish. I would leave at least one intact for the clownfish (it's the perfect anemone replacement for them).
I would also add a skimmer - it won't skim much but keep the water well oxygenated.
If done this way the fish will be clean after 2 weeks, but it is safest to go at least 4 weeks before raising the salinity.
The salinity has to be raised slowly. At the beginning raise it no more than 0.001 per day. When you reach 1.016 you can go faster (I've dripped fish from there to full salinity in an hour with no ill effects) or keep it there for the remainder of the fallow period.
Ideally, before adding the fish back to the DT you should run the fish one by one or in small groups through TTM to be sure they won't have any hidden ich on them.
In your specific case, if you keep the tang, you may have to do it slightly different:
- Take all inverts out and store them in the 5 gallon for 11 weeks.
- Store the live rocks in a clear plastic container of appropriate size (may need a heater and for sure a small pump) for 11 weeks
- empty and clean the 65 gallon (ideally sterilize it with bleach, but don't forget to rinse it well and to add a chlorine neutralizer like Amquel or Prime) and set this up as your treatment tank.
- After 11 weeks put the rocks and inverts back in.
This will leave you with a remaining risk that some ich may have slipped through, but is pretty much the only way to do it without getting an additional treatment tank/container (20 gallon would likely be needed).
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