Ifa fish is so heavily infested with ich, and I mean really bad and you think death is immenent, I would consider a freshwater dip. Only as a last resort. It can help the ich drop from the fish and help him breathe easier, as ich attaches itself to his gills and causes the fish alot of stress just trying to breathe. BUT if you have just a few ich dots I would pass on the dip, it can hurt more than it can help. The dip will stress the fish out badly and should only be considered a last resort, at least in my opinion. Removing the fish, all the fish even those not showing signs to a separate tank and treating with hypo salinity is a great option. You could do that or treat with copper. Just have to have a copper test on hand so you know you arent poisioning the fish and making matters worse. It also will show you that you have the proper amount of copper in the tank, too little and you are wasting your time. Hypo salinity will help the ich to fall off the fish and give him fighting chance. I would not do both though. If using hypo treatment a refractometer is the best way to monitor the salinity. I have not had ich myself but have read enough on this very site to know what needs to be done. I believe you can dropthe salinity pretty quickly,but its the raising of it that requires it to be done slowly over several days.
Keep in mind you have to leave the fish in treatment for 8-10 weeks to be absolutely sure it is gone. That means NO new fish in the tank during that time as well. It can take ich 8-10 weeks to be completed killed off in the main tank. Better safe than sorry. Last thing you want to do is rush it and have it come back stronger than it was.
Good luck to you both!
180g Reef - 60g Rimless Reef -90G FOWLR- 125g Malawi Cichlids- 40b REEF- 34g Fresh Planted-working on- 20L FOWLR- working on
I have a mix of many different saltwater fish amongst my tanks, but I love my Tangs most of all.