I started a 15 gallon reef tank about 3 weeks ago just using tap water with a water conditioner and everything is doing fine other then alge and I am aware i need RODI water. If i start doing water changes with RODI will it filter out my tap water ?
Use RODI and you wil be good. Start with a small water change and gradually increase to about 30% then you should have removed a lot of the nasty stuff. Each water change means more contaminants taken out. Use a solid DI filter and use a TDS or EC meter if possible to measure purity