The water that comes out from our taps may be filled with all sorts of toxic materials, such as lead, mercury, arsenic, chromium, pesticides, industrial pollutants, etc. These pollutants are toxic to humans as well as aquarium fish, reef corals, and invertebrates. In marine & reef aquariums it is essential to maintain low levels of phosphates, silicates, and nitrates to keep undesirable algae from overtaking the system, yet tap water in many places is high in these compounds. The solution is Reverse Osmosis purified water, and for reef aquariums, an additional stage of deionization resins.