depends onm how big your tank is, if it is small then use something like jungle, lifeguard; but if you have a larger tank say 55 gallons or more the use something like coppersafe.
Keep your fish healthy by clean water, adequate water changes, enough space, minimal stress and the correct temperate and you should never encounter ich. I have NEVER experianced the disease in any of my tanks despite not putting fish through a quarentine period.