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Reverse Osmosis Filtration for Aquarium Use

This article will explain the reverse osmosis water filtration process and how it relates to the aquarium use. A reverse osmosis filter is not used to filter the aquarium . It is used to purify tap water for use in filling aquariums and changing water. As you will see, this pure water must be "reconstituted" to make it suitable for aquarium inhabitants. RO water is used for soft freshwater species, marine and reef aquariums.

A Brief Look At Aquarium Filtration

In any aquarium the primary purpose of the filter is to remove harmful fish by products. Ammonia is given off by the fish and food in the aquarium. In the filter specialized bacteria consume ammonia as a food source. Their byproduct is nitrites, deadly to discus, reef corals, invertebrates and most fish. However a second type of specialized bacteria consume nitrites as a food source. Their end product is nitrates. Nitrates are not as deadly as nitrites, but do cause developmental problems in fish. Nitrates are removed by water changes. Plants consume nitrates.

The efficiency of a filter is determined by the flow rate and the type filter media provided to colonize these "nitrifying bacteria". In any power filter for a discus tank, bio media should be added. It may not be sold as part of the actual filter. Any brand of power filter will work well for discus if good bio filter media is added. It is usually sold as bio filter media for canister filters. Some is made of sintered glass, some is lava rock. All good types of bio media have two things in common. They are inert and very porous, allowing lots of filter bacteria to colonize them. Avoid hard ceramic rings as they are not porous. While a wet dry type filter is great for degassing (removing carbon dioxide and oxygenating the water) in a discus tank, the plastic bio balls are not an efficient bio media. If using a wet dry filter, add some good bio media in the "wet" portion of the filter (submerged).

Canister filters and fluidized bed filters are closed system filters and actually place a load or demand on the oxygen in the water. In other words the fish compete with the filter bacteria for oxygen. In the event of a power outage these type filters can rapidly produce extremely deadly hydrogen sulfide gas. This is referred to as "going anaerobic". The filter bacteria have died off and the by product is this deadly gas. One major canister filter manufacturer has recognized this and has a very nice "wet dry" canister filter on the market that oxygenates the water. It takes several weeks for these filter bacteria to become "established" in the bio media.
There are products on the market to speed up this process of "cycling" a filter. Most discus breeders use air driven sponge filters as they are economical, very porous, efficient and work on a central air system. Active bio media should be cleaned only in aquarium water periodically as the chlorine in tap water can kill the bacteria. Cleaning a filter too frequently can disrupt the bacteria. In a properly functioning filter, ammonia and nitrites should not appear when testing the aquarium water. Nitrates will show up. Some tap water supplies contain nitrates. It is a good idea to test yours.

Water changes are done to remove nitrates and impurities from the aquarium water. High nitrates indicate an aquarium in need of more frequent water changes. With a properly functioning bio filter, the average discus aquarium will do fine with twice weekly water changes of 40%. Large daily water changes are needed when medicating a tank, growing out small fry, overfeeding, having a high stocking density or when ammonia, nitrite or nitrate levels are high. There are many differing opinions on this. As you can see, nothing is etched in stone as many variables are involved. Discus aquaria require frequent water changes but will do fine on a regular schedule that does not take the fun out of it. The reverse osmosis filter is used to purify water used in the water changing process.


Reverse Osmosis is a method of filtering water by pressing the water against a semi-permeable membrane that sits inside a filter housing. This membrane allows water molecules to pass through, but not others. Minerals, trace and other elements are removed producing pure raw water. This water is too pure for aquaria use without reconstituting. Marine hobbyists add salt mixes containing all the essentials. Soft wateraquarists must also add trace elements and minerals back to the water to make it suitable.


The water passes through several filters, each representing a stage.
STAGE ONE : Water enters the filter through a micron sediment pre filter. This removes small particulates, bacteria, cysts, spores and grains of sand.Some sediment prefilters use carbon and some do not.
STAGE TWO : The water then passes into the carbon filter. This removes chlorine, toxins and chemical impurities along with any pipe cleaning additives used by the water department. It is crucial that a high quality carbon filter is used for this stage. Special carbon filters available today are capable of removing the new generation of chloramines found in today's tap water, a mix of chlorine and ammonia that many municipal water suppliers now use as disinfecting agents in tap water supplies. However, this new technology is not available on standard ro filters sold on the market today. In time it will be. Osmotic does offer the chloramine filter on one line of our ro filters.
STAGE THREE : Next the water passes through the membrane to complete the filtering process. There are different grades and types of membranes. TFC. membranes are thin film composite. They are suitable for well or chlorinated tap water supplies. Chlorine can damage them but a good carbon prefilter solves this. In the past distinctions were made on membrane application based upon whether it was to be used on a chlorinated tap supply or untreated well water. CTA membranes,cellulose triacetate, can handle chlorine but they are more costly, less efficient and not necessary. The vast improvements in carbon filters render these obsolete in every day use.
The best membranes on the market are tested and certified by the N.S.F. and approved by the F.D.A. for drinking and food use. They feature high removal rates of silicates, phosphates, nitrates and other compounds. Contaminates removed by the filter are discarded through tubing going into a drain. The "reject" brine water used in this is the equivalent of taking a couple extra showers per week or washing the car. The reject water is not similar to turning on a faucet. It is a small stream going through 1/4" tubing.
Reverse osmosis water treatment is much easier to use and more economical than the other two methods of water purification, which are deionization and distillation. RO water can be "polished" with di as an extra stage after the ro for sensitive reefs requiring totally pure water. These filters are called rodi filters. In this case the di filter is not quickly exhausted as the ro membrane has done most of the work.
Pure product water (ro water) is delivered through tubing coming from the membrane. It drips out at a rate determined by the size membrane. A 50 gpd ro will produce close to 2 gallons per hour under ideal conditions. Performance is based upon water pressure and supply water temperature. In low pressure situations a booster pump is used to increase pressure. The ro tubing coming from the membrane can be attached to a float in a sump or barrel, to the ice maker on the refridgerator, to a drinking water tank or any combination.
An auto shutoff valve is used in this. You can tee off to as many devices as desired. However this must be configured properly. When adding floats on a drinking water system special considerations arise which must be addressed properly including the addition of a a check valve to prevent back flow from the barrel. When using a float, the auto shutoff valve must be used to prevent harmful back pressure on the membrane. Without a shutoff valve the filter must be manually turned on and off for each use by turning on the cold water supply.


First step The filter is attached to the tap water supply. Most RO filter systems feature a high quality chrome diverter that will attach to any sink in any house. To operate the filter, you turn on the cold water and pull a small lever on the diverter. This sends the tap water to the unit. Simply turn off the water to stop the operation. There are several options for attaching the tap water supply to the filter.A garden hose fitting or saddle valve may be used.
Second step- you place the reject brine tubing into the sink drain. This carries impurities down the drain. If attaching the filter to a saddle valve on the cold water supply, the brine reject line can also be attached to a saddle valve on the drain line.
Third step-you place the ro product tubing into a barrel or container to hold the water, or attach to any floats or devices if using the auto shutoff valve.
Next, after all tubing is in order you turn on the cold water supply. This places the filter in operation. The efficiency is determined by the temperature of the water and the pressure of the supply water. Cold water only should be used on the filter.
Because the unit is rated in output gallons per day (24 hour period) , water does not pour out of the filter. RO water is produced in a slow, steady dripping stream. Pressure is usually not a problem on city water supplies. RO membranes require a minimum of 60 pounds of supply water pressure to operate at rated efficiency. On well systems there is a simple device to boost pressure on the filter. There are many ways to install your unit. It easily fits under the sink when used as a drinking water filter. The supply of tap water can be fed with a self piercing saddle valve or a valve that goes in line on the existing cold water shut off valve valve for the faucet. A saddle valve is also available for the drain line. The product tubing can run up or down, quite a distance to any location. When the auto shut off valve senses a demand it places the ro filter in use. When the float closes the water flow, the shut off valve stops the filter and eliminates harmful backpressure in the membrane.


They are always on complete drinking water systems. They are not a component found on ro filters but are sold as accessories. Many tropical fish hobbyists order reverse osmosis filters from mail order companies that refer them to the manufacturer for the add on auto shut off valve. Installing the auto shut off valve involves cutting all tubing and installing the tubing in a precise configuration. Some manufacturers offer a line of ro filters with the auto shut off valve preinstalled as standard equipment. For barrel storage a float is added. Anytime a reverse osmosis filter is used for tropical fish, marine, reef or Amazon River type aquariums, a membrane back flush valve should be installed. High use demand is very different than keeping a 3 gallon drinking water vessel filled.


When the ro filter is used strictly for drinking water a light demand is placed on the membrane. For instance a 50 gpd membrane may keep a 3 to 5 gallon tank topped off as water is used. However, when the ro is used for other applications, and larger storage barrels are used and frequently depleted of water, the membrane is placed under a heavy demand. In this case it is advisable to back flush the membrane on a daily basis. The back flush procedure must be performed with the ro filter in service while it is actually producing ro water. This simply involves opening the back flush valve for 5 minutes to flush accumulated mineral deposits off of the membrane. Remember, water is pressed against the membrane and pure water passes through while solids (minerals) are eliminated by the brine reject line. With time, suspended and precipitated solids accumulate on the membrane. This reduces membrane efficiency. Back flushing removes much of this deposit. This will ensure optimum life for your membrane. Anytime a reverse osmosis filter is used for tropical fish, marine, reef or Amazon River type aquariums, such as discus aquariums, a membrane back flush valve should be installed. The membrane back flush valve is standard equipment on Osmotic Reverse Osmosis Filter Systems.


This varies depending upon demand and water conditions. As a rule of thumb, the sediment prefilter cartridge should be replaced every 4 months. The sediment filter works from the inside to the outside. When discoloration is visible it is time to replace it. Some ro filters feature a uv resistant clear sump housing which allows cartridge condition to be checked at a glance. The carbon block prefilter should be replaced every 2,000 gallons. Keeping the sediment filter clean protects the carbon block from clogging. As filters become exhausted, system efficiency and ro water output declines. The membrane replacement schedule is variable. On a drinking water filter the average life is three years. On a system used in aquaria the life of the ro membrane is shorter due to the higher usage. Reverse osmosis filter membrane life is prolonged by using a membrane back flush valve. This valve bypasses the flow restrictor on the unit and allows the accumulated debris on the membrane to be flushed away. The back flush valve is opened for a couple of minutes on a regular basis. If the ro filter output sharply declines even with new prefilter cartridges, it is time for a membrane replacement. The harder the water in your area the faster the ro filter membrane is exhausted. The condition of the ro filter membrane can be tested with an electronic pen tester. As the membrane becomes exhausted, the TDS (total dissolved solids) will rise. It is normal for ro water to have some change in ph as tap water can vary in ph and changes dramatically twice a year in many areas.


Yes. The 150 gpd membrane will fit your existing membrane housing. It is physically the same size as the smaller one but is much denser. You can upgrade as easily as changing the membrane and flow restrictor fitting. The flow restrictor meters the flow to match the membrane. An existing reverse osmosis filter can easily be upgraded to 100 or 150 gallons per day output. A membrane back flush kit should be used with the ro filter membrane upgrade.


For marine and reef applications the rodi filter will give you the purest water to work with. For Amazon River aquariums, such as the discus aquarium, it is not necessary but some choose to use it and then reconstitute to the desired parameters.


RO water should be used whenever soft pure water is desired. In marine and reef aquaria, the ro water is blended with salt mixes appropriate for the intended use. This adds trace elements, salts and minerals necessary to maintain life. Using pure water eliminates the cause of many unsightly problems encountered with tap water. Impurities, including phosphates are removed. Many of these impurities provide nourishment for undesirable bacteria and other critters. For discus and other soft water fish, the ro water is reconstituted. This means adding the trace elements and minerals back into the water.
For discus breeding, the water can be reconstituted using commercial products until the desired hardness is achieved. Tap water that has been purified by carbon filtration can be used to reconstitute the ro water. The filtered tap water is carefully blended with the ro until the correct hardness is achieved. This will give you softer water with a proper ph level. This method is fine for growing and maintaining discus. You can use a measuring cup to determine the ratio of tap to mix with ro. It is not necessary to blend the ro water and tap water in a barrel. When changing water, you can add the proper amount of ro water to the tank followed by the tap. If the tap is unfiltered a dechlorinating agent must be used. These contain salts and often buffers are added to them which can alter the water chemistry, possibly defeating the purpose of using ro. Use care in choosing which dechlorinating product to use. The obvious choice is to remove chlorine and harmful substances from the tap water by carbon filtration. Python type refill tubing can be plugged directly into the carbon filter and used at temps suitable for discus. Tap water filters are inexpensive and remove major impurities from the water including disinfecting agents such as chlorine. Care must be used to select one that is rated for warm water.

RO water should be stored in a food grade drum, large size ro drinking water tank or large water container approved as food grade. Food grade barrels are a little more expensive than trash cans or other containers, which may have various chemicals including mildew retardants added to them during manufacture. Garden hose is unsafe due to the chemicals used in many of them. Some may be okay but it is a risk not worth taking. The storage container should have constant aeration to prevent stagnation. Heat should be provided.
Once a month clean the barrel and any delivery tubing with a very dilute vinegar solution and rinse well. This should also be done to canister housings when replacing cartridges. Never add tap water to the ro barrel. Never add dechlorinating products to the barrel. You do not want the nitrification process starting in the storage vessel as the water will contain nitrates. When using the blend method, pump the ro into the aquarium followed by the tap. A pump with tubing can be used to deliver the water from the storage barrel to the aquarium. Never use a garden hose. Use clear poly tube. The clear tubing can easily be cleaned periodically by submersing for a few hours in a 5 gallon bucket with dilute bleach. Attach a small power head to the tubing to circulate the water through the tube. Repeat with tap water having a high dose of dechlorinator added. Bleach contains chlorine. Rinse well and air dry.
Note: In the past, 100 gpd membranes were large and bulky and did not fit standard size housings. Due largely to demand from aquarist use, manufacturers now make 100 gpd ro membranes and even 150 gpd ro membrane that easily fit standard size housings. In the past, a 100 gpd filter involved stacking two 50 gpd membranes into "piggyback" fashion. When membrane replacement time rolled around you had to obtain two membranes. The recent advances in component manufacture make ro filtration an economical solution to water purification for aquarists. I hope this has given you some insight into the area of using ro filtration in aquaria applications.

Copyright 2001, Al Johnson, author. Al spent many years working as a plumber with specialized training and state certification in back flow prevention device installation, testing and repairing. This article may not be copied or reproduced without written consent of the author.