Both salt and fresh water aquariums need filtration systems. Without one fish and plant waste builds up in the water and creates a toxic environment. Basic tap water contains impurities that can throw off pH levels and contains harmful chlorine. Reverse osmosis (RO) systems filter out fine mineral particles and harmful chemical additives. This allows for safe fresh water to be circulated to the fish tank. RO filtration generally uses four filtering stages. The first cylinder removes larger sediment particles, the second is a granular carbon filter, the third is a block carbon filter, and finally a spiral wound membrane filter. Each step reduces finer and finer contaminates. RO water can be so pure that you have to add minerals essential to marine health back before it is tank ready. These systems tend to be very low maintenance with simple carbon filter changes need every six months, and total RO membrane filter replacement needed only every few years. These systems are available for both household and large commercial aquariums.