Water Purification and Top-Off by Mike Paletta

“Garbage in: Garbage out” is one of the long held dictums of computers. This is also true of reef aquariums. Unfortunately, one of the most overlooked aspects of reef keeping is the quality of the tap water that is being used in a reef aquarium. Fish and corals are not only composed almost entirely of water, but they also live in it and exchange the water within their own bodies almost constantly. As a result the quality of the water in which they reside plays a vital role in their long-term existence. Therefore, when setting up a reef tank, it is essential that the water used be of the highest quality. Although most local water is more than fit to drink, it may contain compounds that are deleterious to the health of fish and corals over time. Not only does most tap water contain chlorine or chloramines as residues from water treatment and purification, it may also contain trace amounts of heavy metals, pesticides, herbicides, phosphate, silicate, and nitrate. If these are not removed or reduced, they will accumulate in a reef tank over time, producing toxic effects for some invertebrates.

Most of these compounds are present at relatively low levels not readily detectable by the consumer. However, It is easy to find out what is present by simply asking the local water authority for a listing of what is in the tap water. Most authorities will provide this readout for free. It usually lists the average level of up to 30 different compounds in the local tap water. Some compounds such as carbonates, bicarbonates, or iron are usually not problematic, and even if their levels are relatively high they should not present a problem. However, if there are detectable levels of any heavy metals (copper, lead, tin, etc.), silicate, or phosphate, then it may be necessary to employ a purification device for the tap water.

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The most commonly employed devices for this purpose are either reverse osmosis (RO) or deionization (DI) units – the most useful being units that contain both stages. A reverse osmosis unit works by forcing water under pressure through a semi-permeable membrane. The membrane acts a filter for molecules above a certain size. As a result, it keeps large molecules such as colloids or proteins from passing through. These waste molecules are then flushed out with the wastewater that this type of filtration produces. The higher the pressure used to pump water through these units, the less wastewater and the more useable water produced. These membranes can be damaged by long-term exposure to caustic agents such as chlorine or large molecules so the water reaching them should first go through a sediment or carbon filter. Employing these relatively inexpensive pre-filters can protect the more expensive RO membrane.

Reverse osmosis is typically employed with the purified water flowing through a deionization unit after it. This unit acts to remove any small ionic ions that pass through the membrane. The unit usually contains a mixed bed of anion and cation resins that act as sort of magnets to trap ions within the unit. This unit can employ resins that change color as they are exhausted. If these are not used, a meter that electronically measures total dissolved solids can be employed. The use of this inexpensive meter allows for the resins to be changed at the proper time. In addition to the mixed-bed resins it is also possible to get cartridges specifically designed to remove certain ions. These include silicate, which is a problem in the southeast, and ammonia, which can be problematic if chloramines are being used in water treatment.

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Since the purification takes some time, it is usually a good idea to have a reservoir in which to store the purified water. This reservoir can also be used to top off the tank as water evaporates. Salt does not evaporate with the water, so only fresh water needs to be added as water evaporates. Many new types of level controlling devices, both mechanical and electronic, are now available. Both work on the same principle that when a tank or sump’s water level drops below a certain level they activate an AC outlet that causes a pump to turn on and fill the tank back to the desired level. They are quite simple to install and use, and they are worth the investment as they reduce the need to haul buckets to fill the tank and they protect the pump from ever running dry and burning out. These devices used in combination with a RO/DI unit provide clean pure water constantly and with little effort.

Mike Paletta is the author of The New Marine Aquarium and Ultimate Marine Aquariums. He has been in the hobby for over 15 years and has written numerous articles for Aquarium Fish Magazine, Tropical Fish Hobbyist and Aquarium Frontiers.