A freshwater dip in my opinion does nothing but stress the fish. It is useful if the fish is covered in ich as it will cause them to drop off but thats about it.
As stated, the best and only way to get rid of ich and be sure of it is hyposalinity or copper in a separate tank. You dont have to go crazy with another tank either, just a heater and some sort of filter would be fine. No rock or sand is needed. It can be done very cheaply if you look around.
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.