As stated above, garlic will NOT cure ich. It may help the fish deal with it better but it is not a cure unfortunately.
Hypo and copper are the only two KNOWN methods of killing ich completely. You may get lucky and the fish may fight it off but it can still lay dormant in your tank and can for sure re-infect the fish, thats why it's recommended to remove the fish and let the tank sit for 6-8 weeks. This will ensure that any ich in the tank is killed and will make it safe again for the fish.
How big is the tank they are in now? Can you catch them to put them into a qt? Depedning on the size of the tangs you may want to go with a rubbermaid tote as it will be big enough for the tangs.
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.