Andy
Before you or anyone else reads this I am not looking for an arguement on this and I am simply going to give my point of view and my experience with this.
First make sure you move the snails crabs and shrimp to the tub as well, before attempting that technique, and I think with some reading you will find that is only half of the recommended treatment . How bad is the ich? If they are covered you will most likely need to take the fish out and quarantine dip them in copper, as bad as I hate to say that. Also you will need a tank large enough to safely house the tang if you take this approach or you will not do yourself any favors and a smaller tank would only stress the tang more and futher his sickness which would worsen the other fish as well. If it is a few spots they can beat it if they are eating and you put a little garlic juice in their food along with something like selcon. The garlic has been shown to increase there appetite, and make them eat, and that is where a good diet with selcon (vitamins) will kick in and help the fish build up their immune system. Also you really should find out what is stressing the fish. Maybe water parameters? Maybe post shipment stress? Maybe one fish is stressing the other? Until you solve that issue you will not beat ich it will come back time and again. If your fish are covered act swift or lose the fish, also don't be fooled if they look all better after they have it the first time, they fall off as part of the cycle and multiply in the sandbed. You need to get a hospital tank if they are really bad and dip them with a copper base dip. If they are not covered you may only stress them worse and worsen the issue. I have tried treating them and I would say its 50/50 when you put them in a hospital tank at least it was for me. I will also say that any fish I had covered that I tried a fresh water dip on didn't make it. Also any fish I had that wasn't eating didn't make it. The copper is really the only thing that will kill the ich which is actually not ich as it is in fresh water, but cryptocaron irritans in saltwater. As for in my tank I have had every fish at one time or the other show a few specs of ich. I increased vitamins, fed mysis in the morning and a good mix of dry in the evening both are soaked in garlic and selcon, and removed any stressors that I seen. I have not lost a fish to ich in my 90 using this technique, Knock on Wood. I will get taken to the shed for this by many but I will say that I am one of the guys who believe crypto is always there its just not active until stressors show up in the tank and healthy fish beat crypto much like humans do the flu and other viruses. Good luck and I wish you the best in treating your fish.