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  Cardinal Tetras:
A School of Beauty

A Multipurpose Tool For Producing High Quality Water

By Mark E. Evans

In the natural world, objects frequently have more than one purpose. For example, a Damselfish comes to a sudden stop in the water with its pelvic fins flared downward which makes it easier for the fish to slow. A Goby comes to rest on the bottom and the shape of its pelvic fins enable it to cling to a rock more securely. The pelvic fins in both examples are homologous structures but they clearly perform separate functions. Similarly, in the equipment-filled world of the aquarium hobby, the reverse osmosis  (RO) filter is a tool that is also capable of performing multiple functions. The intent of this article is to provide further clarification on several of them. A reverse osmosis filter is undoubtedly a useful piece of equipment to own, but before you purchase one you first need to decide what you need it to do.

Most aquarium hobbyists have seen ads for RO filters online, in magazines or in catalogs. But what did they take away from viewing such ads other than prices and promises of purer water and a healthier aquarium? The expense alone of buying an RO filter demands that you do your homework before purchasing one. The fact is that not all filters are equal and ads alone are just too little information to go on. You need to decide which RO filter is correct for your particular application. The general design of most RO filters is basically the same but the filter cartridges and membranes that come with the various models differ greatly and these are the items that make up the majority of the price of an RO unit. So, the replacement parts are an expense you should consider prior to your purchase of an RO unit because not all the filter cartridges that come pre-installed in an RO filter unit may suit your needs. And replacing them unnecessarily or prematurely is a costly lesson that can be easily avoided with a little planning.

A very typical four stage aquarium use RO unit with a sediment (left) and two carbon (center and right) pre-filters.  The membrane is the white cylinder across the top.  The clear housings on the pre-filters makes for easy inspection.  Note that the sediment filter is past due for replacement!  Photo Adam Cesnales

RO Unit Components

Before we discuss choosing a unit, a brief discussion on the various components of an RO filter may be helpful. RO units usually consist of several individual filters placed in series.  Each individual filter is referred to as a "stage".  The placement of the individual filter cartridges on RO units may differ according to the manufacturer but the general path of the water through the filters remains the same. 

The water begins its journey at the source, for example a pipe under you kitchen sink. It is then pre-filtered through one or two cartridges that remove substances that could damage the RO membrane (which is the most expensive element of the RO unit). The first is usually a sediment cartridge that removes particulate matter that could clog the membrane. The second is a chlorine pre-filter. The chlorine or chloramines used to disinfect municipal waters damage some types of RO membranes (particularly the TFC - thin film composite type), so it is necessary to remove such chemicals prior to the water reaching the surface of the membrane. (The carbon filter also removes heavy metals and many chemical pollutants). Once the water is pre-filtered it is ready to be pushed through the membrane itself. This is accomplished through back pressure, hence the name "reverse osmosis". The pressure is created by a small part called a flow restrictor, which is connected to the waste water line.

The most common configuration of stages (in order of water flow) is sediment, carbon, carbon, RO.  The presence of two carbon filters demonstrates how important it is to remove chlorine and chloramine. Drinking water units often have a final carbon filter for polishing the water and many aquarium units have a de-ionization filter for the final stage. RO membranes alone are not capable of removing 100% of the impurities from the source water, so a de-ionization cartridge is placed at the end to filter out most of the remaining traces.  

The least expensive models of RO units, sometimes called "bare bones" units usually consist of 2 stages: a carbon block filter to remove sediment and chlorine, etc., and the membrane itself.    [Editor's note:  Some companies sell "bare-bones" units without any pre-filter at all.  These units are doomed to a short life span.  Choosing units with at least a single prefilter will greatly extend the life of the membrane and is cheaper in the long run.>

RO Unit Function and Performance

The back pressure produced forces some of the water through the membrane and allows the rest to pass by into the waste water line. Depending on several variables, anywhere from about 1 to 6 gallons of waste water are produced for every 1 gallon of filtered water collected. The waste water serves two important purposes in an RO filter. It flushes away the substances extracted from the source water and it produces the very back pressure that allows the filter to operate.

The primary influences on the type of RO filter unit you will want to purchase will most likely be geography and geology, because where you live invariably determines what type of water comes out of your tap. For instance, cities that receive municipal water from the Mississippi River usually have very hard tap water. Cities in Southern California have the same problem. Water is the universal solvent, so rivers pick up all kinds of substances as they travel through geologically rich areas. For the home aquarist this can be a bane. Very hard water (high in minerals like calcium and magnesium) is often incompatible with breeding freshwater fish that are native to very soft waters. (Examples are Discus, Cardinal Tetras, and Corydoras Catfish). This difficulty leads many aquarists to finally buy an RO filter.

Choosing an RO Unit

If reducing the hardness of the water is enough, this can be accomplished with a 2-or-3 stage RO filter or even a bare-bones unit.  These units are economical and are sufficient to filter water for an aquarium system that is relatively undemanding. The output rate on most of these bare-bones units is low (around 25 gallons per day or less), and the water may test from 10 to 20 parts per million of dissolved solids, certainly good enough to soften up any liquid rock that may come out of your tap! But there are other more expensive 3-stage units that are capable of filtering larger amounts of water and reducing the dissolved solids level to about 10 parts per million. When deciding which filter in this category best suits your needs, it is important to know the sediment level of your tap water and how the water is treated by your local municipality. Water that originates from rivers can have high levels of particulate matter, so the size of the sediment pre-filter and carbon block filter on your RO unit will affect the unit’s performance, and determine how often you will need to replace the individual filter cartridges.       

If you live in a rural or agricultural area your main source of water may be a well. Ground water is typically a hard water source, and in addition it is easily contaminated by local runoff from agriculture and industry. It is therefore not uncommon for a well to be a soup of unpleasant substances that affect water quality. Although it may be certified perfectly palatable for human consumption, the water may fall short in being suitable for keeping aquarium fish. Pollutants such as nitrates, phosphates or heavy metals may be a problem for your fish. In this case, an RO filter equipped with 4 or 5  stages might be helpful in eliminating these substances. It is worth considering doubling up on chemical filtration by having two carbon pre-filter stages, and a de-ionization filter at the end to polish the water to a high purity.

Lessening the growth of algae is the goal of many aquarium hobbyists and utilizing an RO filter for this purpose is now a common practice. Excess algae growth is part of the freshwater and marine aquarium hobbies alike. It is becoming more common for local water authorities to treat tap water with orthophosphates in order to prevent corrosion and scale formation in pipes. Unfortunately, this phosphate overload strikes freshwater aquariums with the plague of planktonic algae, turning clear water to green pea soup. In marine aquariums, high phosphate levels are even more problematic especially when corals are being kept. Excess organic chemicals have a deleterious effect on the growth many captive invertebrates. Alleviating the situation with RO water is simple as long as the correct filter elements are used.

Stand Alone DI Filters vs. RO/DI

Stand alone de-ionization filters are available, and DI water is more pure than RO water, but in most cases they are not worth the expense in the long run. The large capacity models have resins that can be reused, but this requires the handling of dangerous chemical solutions. Also, they are only capable of producing small quantities of water before the filter cartridges are exhausted. The tap water purifier marketed by Aquarium Pharmaceuticals is an example of a small de-ionization filter that uses disposable mix bed resin cartridges. The cost of each cartridge is around $20, but they filter less than 50 gallons per cartridge before becoming exhausted. Certainly, this is an impractical solution for anyone wanting to produce purified water for a reasonably sized aquarium system.

On the other hand, passing water through a DI stage after it has passed through an RO unit produces water with DI purity, but since the RO unit leaves so little for the DI stage to remove, it lasts much longer than stand alone DI units.

Algae growth is connected to nutrients like iron, nitrogen, silica, as well as the previously mentioned phosphate. Most of these elements are very effectively removed by RO, except for excessive Phosphate.  If the source water is high in phosphate, a de-ionization stage on the RO filter may be required to reduce it to reasonable levels. If  you decide on an RO unit with a de-ionization stage, you might want to consider running a second de-ionization cartridge in series. Single DI filters must be changed before they are completely exhausted (indicated by change in color).  Running two de-ionization cartridges together is actually more effective because of the increased contact time the water has with the resin, and it saves resin since the first DI cartridge can be run until it is completely exhausted since the second will continue to work.  When the first cartridge is exhausted, the second can be moved to the first position and a fresh cartridge placed second. This way the RO water produced will always be of the highest quality for your battle against the dreaded algae growth.


When choosing an RO unit, think carefully about the rating of the membrane. Membranes are rated in gallons per day. However, the rated output can only be achieved  under ideal conditions of water pressure and temperature.  Most residential water is too cold and at too low of a pressure to even come close to the rated output. For example, a membrane rated at 35 gpd may in fact only produce 10 to 15 gpd. So, anticipate how much water you intend to use per week or month and purchase an RO unit with a membrane rated higher than what you require. A membrane will last for years, so replacing one prematurely because it is underperforming is a waste of money. Excess water from your RO unit can always be stored in a plastic garbage can, but if you are ever caught in a situation in which you need to make large water changes in your aquarium, it can be a long wait for your RO unit to produce enough water.

RO units are versatile household appliances; they can be setup to produce water for an aquarium system as well as drinking water for a family. And ideally, the waste water can be redirected from going down the drain and used for watering a garden, washing clothes or even washing a car. Aquarists at all levels of experience can benefit from the investment in a good reverse osmosis system! Start Using One today and your animals will thank you tomorrow!

Tap water Filtration: Reverse Osmosis, Deionized, Distilled Water... on WWM

Related FAQs: RO/DI & Distilled Water 1, RO/DI & Distilled Water 2, RO/DI & Distilled Water 3, RO/DI & Distilled Water 4, RO/DI & Distilled Water 5, Rationale, Selection, For Commercial/Large Output, RO Water Storage, RO Water TreatmentMaintenance/Repair, Deionizing Source Water Filtration, Kati-Ani DI Units, Kold-Steril Units, Water ChangesWater Make-up, Nitrates

Related Articles: Water Purification Using Reverse Osmosis. The Soft Water Aquarium: Risks and Benefits by Neale Monks Water ChangesWater QualitySynthetic or Natural Seawater, Nitrates


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