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Reverse Osmosis - RO Filter Guide

Why do you need an RO filter?

Most natural tropical waters have a water hardness of GH 4-5. European water hardness is different from country to country, but there are many places where we have really hard water (GH 15-25 or more).

If you live an area that has hard or very hard water or you are not satisfied with the tap water quality in your area (or you are planning to make a saltwater fish tank), you have the possibility to produce water that has similar composition with that in the tropics or at sea. You are going to use RO filter to do that. The RO abbreviation stands for Reverse Osmosis. This relatively simple technology does not need electricity, it uses the tap water pressure to "squeeze" the H2O (water) molecules through a membrane that will let nothing else through... You will use this clean, purified water in your aquarium - after some treatment. Besides the clean water outlet there is another wastewater outlet hose on the device - we call it the "bad water outlet". This will drain very hard water that contains all the salts that were rinsed from the exterior of the membrane. Place this hose in your sink and never use this water in your aquarium (you can use it for other purposes like watering your plants for example).

There are also pre-filters in your RO device. These are meant to eliminate chlorine, heavy metals and other phisical contamination.

The different kinds of RO filters

RO filters are usually categorized by their efficiency. As a general rule the cheaper filters are less efficient and they produce less water per hour then their more expensive counterparts. This efficiency is measure by a ratio. For example, if you see 1:4 that will mean 1 unit of clean water for 4 units of wastewater. RO filters usually have their daily (!) yield written on their boxes - in liters or gallons. More expensive units can have up to 3 or even 4 membranes, improving the purified water / wastewater ratio.

It is more than enough to have an RO filter with just one membrane by our experience. A Dennerle Osmose Compact 130 (with 130 liters of clean water produced daily) will be sufficient for a 60 liter aquarium because you will only have to produce around 30-40 liters with it - on a weekly basis.

You also have to know that the maximum yield of a device is calculated at 25 degrees Celsius and a tap water pressure around 4.1 bars. This means that your RO filter will almost certainly produce less than what is written in it, because tap water temperature is usually much lower - around 15 degrees - and water pressure rarely exceeds 2-3 bars. Do not worry though, you will have more than enough purified water with time, just make sure you switch the device on - in time. You can also buy a booster pump to increase water pressure and efficiency.

A good example of how the yield depends on temperature change is the Dennerle Osmose Compact 130 which will produce just 55 liters / day at 8 degrees Celsius, but a more impressive 126 liters at 18 degrees.

The main parts of an RO filter

1. Tap fitting. The devices usually come with a 3/4" external thread (!) tap fitting. If you do not have a tap with 3/4" external thread and you cannot connect to other home appliance (washing machine, dishwasher, etc) fittings that have this size then you will have to visit a local plumbing shop and buy the necessary fitting converters (you can also get a tee fitting for example).

2. The tap water inlet hose is connected to the tap fitting element, this hose will lead into the first pre-filter. Dennerle RO filters will show you the flow direction with white marks on the filter houses. This will also show you which hose will connect to the tap water. The water flowing through this inlet hose will get to the activated carbon pre-filter house.

3. As first pre-filtration step, the water will usually flow through an activated carbon pre-filter. This will remove all chemical contaminants (chlorine, heavy metals). The pre-filters will have to be changed regularly (about every year) to protect your membrane. (pre-filters are a lot cheaper to change then the membrane). If your water has high chlorine you are advised to change pre-filters more often due its limited capacity of chlorine removal.

4. The water will then flow into the fine filter (5 or 10 micron pre-filter) this will remove all visible and non-visible mechanical particles. This pre-filter should also be changed every year or twice-a-year.

5. The pre-filtered water will get into the membrane house. Lifespan of RO membranes is usually 3-8 years - if the pre-filters were regularly changed. Neglected pre-filters will drastically reduce RO membrane lifespan. Please make sure to keep the membrane humid if you are not using the RO filter for longer times. Drying out will also mean the end of the membrane life. Dennerle was careful enough to pack some corks that can be fitted on the in- and outlet hoses to protect the membrane from drying out.

6. The membrane house has two hose exits. One of them will produce the purified water, you are going to use this in your aquarium.

7. The other will produce the wastewater, this will leave the membrane house at mark 7A and the hose continues at mark 7B. You will have to connect this end to your sink.

8. There usually is flow restrictor (little white cylinder) on the wastewater hose. Its function is to limit the flow on this hose and improve the efficiency of the RO filter.

9. On bigger RO filters (for example the 190 liters / day Dennerle RO filter) there is a bypass on the flow restrictor. The bypass has a smaller tap, if you open that tap, the system will rinse itself and clean the membrane surface. When the system rinses itself (the tap is open) you will see a stronger jet of water coming out from the wastewater line. This tap should be closed for normal operation (the flow of the wastewater should be weak).

Setting up an RO filter

The device can be mounted on a wall (vertically) with the two clips found on the membrane house, but it can also be operated in horizontal position - there is no difference in efficiency. For wall mounting remove the two holding clips from the membrane container and mount them vertically one above the other at a suitable distance from one another on a wall or board and carefully press the osmosis system into the holding clips. Make sure you operate the device in a room (bathroom, appliance room, etc.) where eventual leakage will not cause damage in the environment and water can drain away beneath the osmosis system.

Connect the tap adapter of the osmosis system to a 3/4" tap (cold water tap!). Ensure that the rubber seal fits properly.

Place the residual water hose and initially the osmosis water hose in the drain (a sink, bathtub or toilet). Open the tap water carefully (not the whole way) and check all joints, connections and filter containers for eventual leakage. Make sure everything is watertight. In case of any leakage stop the water and fasten the joint by pushing the hose deeper into the joint or by fastening the filter houses.

Allow the system to run for 2-3 hours in order the rinse away the preservative in the membrane. During this period, do NOT use the osmosis water, but allow it to drain away. If your device has a bypass tap you should open that after 2 hours and wait another 15 minutes for the RO membrane to be cleaned from the exterior. You can close the bypass tap at this point and the osmosis water can now be collected and used.

The residual water can also be used, for example for cleaning, watering your houseplants or flushing your toilet. Recommendation: The osmosis water reservoir should be equipped with a safety overflow to channel the water into the drain. This will enable you to prevent “flooding” should you forget to turn off the system in time. If you place your reservoir into the bathtub you will also be safe in case of overflows (but not from your spouse...)

How much RO water does a device produce and how to use RO filters?

Quality RO filters will remove 95% or more of the Ca, Mg, Na, and other salts as well as nitrate, phosphate, chlorine, heavy metals and other chemical elements contained in the tap water. The clean water produced will be very soft and virtually free of harmful substances. The quality of the water can be measured with GH (total hardness) or KH (carbonate hardness) liquid tests, quick tests or with our favorite, the conductivity (TDS) meters.

Osmosis water is very pure and therefore contains practically no hardness at all, which is necessary to buffer the pH value. For this reason osmosis water should always be set to the required hardness in the aquarium before use. Ideal water parameters of a Nature Aquarium are a GH of 6 (TDS meters will show around 130). You will set the water hardness and mineral content precisely according to the needs of your aquarium fish, shrimp and plants with a GH raiser (GH plus) within seconds.

Other reference water parameters: discus fish: 3-5 GH (1-3 KH), tropical planted aquarium 4-7 GH (2-4 KH), Malawi cichlids 3-6 GH (5-8 KH), Tanganyika cichlids 8-12 GH (16-18 KH).

Please use good quality reef salts or marine salts for coral reefs and saltwater aquariums or please consult the relevant specialist literature to find out about the care needs of specific fish or consult us for specialist advise.

Water hardness categories: soft water has a GH less then 7. Medium hardness: GH 7-14, hard water: GH 14-21, very hard water has a GH with more than 21.

The water production can be stopped by turning off the tap. It can also be automatically deactivated using a standard solenoid valve, which can be installed subsequently in the hose between the tap and the active carbon filter (on the inlet hose).

Care and maintenance

We recommend using the system at least once a week for 30 minutes. If the system has not been used for a period of more than two weeks, the osmosis water should be drained off for approx. 1/4 hour before being used again. This will flush out any bacteria that may have got into the system. If the system not used for longer periods, please ensure that the membrane is not allowed to dry out. To this end it is advisable to plug the hose ends for the osmosis and residual water with a rubber stopper.

Most RO filters are virtually maintenance-free. To ensure a long membrane life, it is important to remove the residues that build up on any osmosis membrane over time by means of regular rinsing. If the membrane is not rinsed regularly, limey surface will become deposited on the membrane surface, which will result in diminished performance and premature failure of the system. It is recommended to rinse the system with the bypass tap (A). If your RO filter does not have a tap, close the water inflow, unscrew the coupling ring from the upper fitting of the flow restrictor (B), remove flow restrictor, and extract the hose insert (little white cylinder) (C) from the end of the hose (to make sure it is not going to be rinsed out with the water into the sink) and open the tap. rinse times depend on water hardness:

If your tap water is soft (below GH 7), rinse time is 15 minutes every 3 weeks. With medium hardness (GH 7-14) it is 15 minutes every second week, with hard water (GH 14-21) it is weekly 15 minutes, with very hard water (above GH 21) it is 5-10 minutes after every usage. If you do not rinse your membrane a slimy coating will appear on it which will limit its efficiency and cause the membrane to deteriorate more quickly.

When should the membrane be replaced? Membranes will be replaced after 3-8 years - depending on usage - proding you have installed a good quality polyamide/polysulfone membrane. If the clean water quality or the efficiency of the RO filter deteriorates than you will know it is time to replace the membrane. Remember that regular pre-filter replacements and regular rinsing will prolong the lifespan of your membrane.

Water quality can be determined by measuring the clean water GH or TDS values. The membrane should be replaced when the conductivity or hardness of the osmosis water is greater than 25 % of the tap water or the amount of the produced osmosis water drops below 70% of the original amount (you have to take note of that when first using your RO filter), despite otherwise correct use (temperature, pressure, filter not blocked). Before measuring these values, the device should have been rinsed for 15 minutes and have been in operation for at least 2 hours.

Replacing the membrane. The top of the membrane container lid can only be unscrewed with a special plastic tool, a spanner. The spanner is a round tool with a handle can be usually found in the RO filter box. Do not throw it out and keep it in a safe location for later use. Make sure that the you store the new membrane under humid conditions (this is why it comes in a sealed plastic bag), never let it dry out!

Please do not open or damage the plastic bag before installing the membrane because it will be difficult to seal it back airtight. Carefully remove the membrane using a pair of pliers. Lightly grease the two O-ring seals on the new osmosis membrane with Vaseline (chemist). Insert the membrane (the two O-rings first) into the membrane container as far as it will go. Likewise, lightly grease the O-ring for the membrane container lid with Vaseline. Screw on the container lid tightly and reconnect the hose with the hose insert.

Allow the new membrane to run for 2-3 hours. During this period, DO NOT use the osmosis water.

Replacing the fine-filter. You will notice whether the fine filter is blocked, because the flow rate will declines significantly. Release the hose screws on the membrane container lid. Turn on the tap and measure the volume of water flowing out of the outlet of the fine filter: Replace the fine filter if the volume is less than 1 l/min. The fine filter should generally be replaced once per year; more often in case of poor water quality.

Replacing the active carbon pre-filter. The active carbon filter should be replaced once per year, in case of heavily chlorinated tap water - that smells of chlorine - every 6 months. Some active carbon filters may release really fine particles that will clog the fine-filter or pre-filter so it is advisable to remove the membrane and rinse the system without it for 30 minutes, avoiding the dust settling on the membrane or fine-filters.

General tips for troubleshooting

In case of leakage find the source of the water and fasten the screw or the fitting. DO NOT screw the fittings too tight as you may damage the thread. Do NOT unscrew fittings once they are under water pressure. All container lids have a rubber seal inside them, you might have to replace them occasionally or use Vaseline for maintenance.


1. Too little osmosis water and too little wastewater. The fine-filter is clogged or one of the hoses is bent, or tap water pressure has dropped. Replace the fine-filter, check the hoses for breaks, or check the tap water pressure (it should be at least 3 bars). In case of weak pressure use a booster pump.

2. Too little osmosis water but a normal amount of wastewater. The membrane has aged or clogged, or the rinsing tap has been left open or the tap water is too cold. Rinse the membrane for 15 minutes, close the bypass tap or try to connect the system to a tap that you can adjust to mix the cold water with hot water (make sure the water is not warm!).

3. A normal amount of osmosis water but too little wastewater. The flow restrictor is clogged or the wastewater hosing is broken/clogged. Replace flow restrictor and check the hoses.

4. The system leaks between the fittings and the container. Carefully screw the fitting in a further half to single turn. If that does not remedy the fault, you will have to reseal the fitting using Teflon tape.

5. The system leaks between the hose and the fitting. Unscrew the coupling ring, extract the hose and check whether the hose end has been cut off evenly and the hose insert is in place. Insert the hose into the fitting with the hose insert as far as it will go (!) and tighten the coupling ring by hand. If the hose connection still leaks, carefully tighten the coupling ring slightly using a SW 16 spanner. It is vital that you hold the fitting in place while doing so.

6. The system leaks at the membrane container lid. Release the hose connector and unscrew the lid. Check whether the O-ring is properly in place or is cracked and if necessary install a new O-ring. Grease the O-ring lightly with Vaseline and tightly replace the container lid. Reconnect the hose.

Central water purification system instead of RO filter

Many customers have told us that they do not nead an RO filter because they have a professional, very expensive water softener installed centrally in their house or office. Our answer is always: No, don't use it. These high performance industrial central appliances do not use the reversed osmosis principle, they will filter out heavy metals and chlorine, which is fine, but they are going to use another salt (usually sodium salts) in higher amounts to "replace" Calcium and Magnesium salts in the tap water. This is OK to drink and certainly good for your home appliances but our experience shows that it is bad for your aquarium (TDS jumps up to 400-600 in water produced by these appliances). You would be better off if you would take the tap water (using a tap that is connected before the water purification system) to change your aquarium water of connect the RO filter on that tap if possible.

On a side note RO water is safe and very healthy to drink and to use for coffee, tea and cooking (some big coffee shops are actually using the same method to produce water for your coffee) but you have to be aware that drinking exclusively RO water and not eating foods that contain Ca and Mg salts (like eggs for example) or other minerals, might be unhealthy for you in the long run as your body will not get the necessary elements...


Using an RO filter is really easy if you follow the basic rules. Most users will just install it and use it for many years without problems. If you do not feel like being ready to install one, you can always purchase one and call for a plumber to set it up for you. The RO filters will not take up much space, they can even be fitted in a kitchen drawer - together with a small barrel for the RO water (with overflow) - this kitchen space should be enough for aquariums up to 180-240 liters and it contains both cold water and drainage pipes.

Also, you can download the Dennerle RO filter guide PDF in 5 languages (German, French, English, Italian and Dutch).

Our friends from Dennerle made a short presentation movie about RO filters. We recommend that you watch this 8-minute presentation, it is in German, but the images are self-explanatory.

If you have questions or comments about the RO filters, please do not hesitate to contact our Customer Support. We would be glad to help!