Freshwater dips will not kill all the ich some will more then likely survive. The only sure fire cures for ich are hypo and copper. Do not use either of these in your main tank it will kill your inverts and coral if you have them. If you do hypo you will need a refractometer to measure the salinity. If you do copper you will need a copper test kit to make sure you have the proper copper levels. Keep your main tank fishless for up to eight weeks to make sure any ich in your tank is dead, they can't survive without a host. You can leave any corals or inverts in the tank since they are not effected by the ich. Good luck