The water that comes out from the standard mains taps is designed for human consumption. To enable this it will have been treated with a range of chemicals to ensure it is safe to drink. Very low levels of toxic materials such as lead, mercury, arsenic, chromium, may be present. While these will have been reduced to levels that are safe for humans they may still be present and will be harmful to fish, corals and invertebrates. In reef aquariums it is essential to maintain low levels of phosphates, silicates, and nitrates to keep undesirable algae from overtaking the system To ensure the water you use in the aquarium is free from these harmful chemicals a process called reverse osmosis can be used to purify the water. For reef aquariums an additional stage of deionisation resins is also recommend.
