Providing filtered water can be one of the most important things you do for your reef tank.  By removing impurities from the water you are also removing fuel for algal growth.  A reverse osmosis or reverse osmosis/deionization system is an excellent way of providing your tank with crystal clear water.  While reverse osmosis systems remove up to 95% of many compounds, they also produce about 30L of waste water for every 3.8L of pure water.  This can be expensive for those on water meters, depending on how often you use it.