It is near impossible to completely have no ich in your tank because whatever thing wet you put in the tank there is a good chance of it containing ich: inverts, corals etc. What you can do to help fight ich is first of all, quarantine fish if you can, feed high quality food, I recommend PE mysis or LRS reef frenzy for frozen food and a mix of various brands of pellets. These should be soaked in Selcon + Vitachem to help boost fish immunity. Secondly, stock wisely: have a good stocking density and always introduce peaceful fishes first to avoid stress. Thirdly, maintain high water quality. Lastly, can consider to invest in a UV steriliser to reduce the ich population in your tank.