Is Reverse Osmosis Water Suitable for Aquariums?

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What is Reverse Osmosis?

Reverse Osmosis, commonly referred to as RO, is a process where you demineralize or deionize water to create purified water. It is a process of pushing water under pressure through a semi-permeable Reverse Osmosis Membrane. The membrane will allow the water molecules through, but will block larger molecules such as minerals and other contaminants.

Many marine and freshwater fish keepers have started using reverse osmosis (RO) to produce high quality water as the membrane traps and removes 90% to 99% of the impurities from the water. The purified water can then be used in the aquarium, while the waste water which contains concentrated contaminants can be discarded or used for your garden plants.

Reverse Osmosis Water vs. Tap Water

Many aquarists have switched to using water from reverse osmosis systems instead of tap water. In most cases, a country’s tap water supply contains too many contaminants which would severely compromise the lifespan of your fish.

Problems with Tap Water


Water from the tap contains a certain number of dissolved solids in the form of salts, phosphates, nitrates with varying concentrations of chloramines and fluorides. While the chemicals added makes tap water suitable for drinking, it can be toxic for your fish. This is especially so in the case of soft-water fishes like the Discus and Dwarf Cichlids.

Due to the nitrates and phosphates, nuisance algae grow will thrive.  While phosphates and nitrates are not harmful to people at lower levels, it can cause a lot of trouble in an aquarium even in very low concentrations.

If you plan on having a reef aquarium, tap water can wreak havoc on your invertebrates and corals. Trace amounts of copper and considerable amounts of total dissolved salts (TDS) can be toxic for your aquarium’s invertebrates and corals. In tap water, your TDS will be in the range of 250ppm or higher which fish and corals are not used to living with these levels of TDS in the wild.

Furthermore, as the water evaporates from your set-up, the TDS levels will increase and could become toxic over a very long period.  Also, the quality of your tap water may vary throughout the year.  This would make it very hard to make consistent batches of salt water and these changing water conditions can have adverse effects on sensitive fish and corals.

Choose Reverse Osmosis Water

Aquatic Life RO Buddie Three Stage Reverse Osmosis

In contrast, reverse osmosis water is free from all dissolved salts, minerals and particles. This is because of the process of Reverse Osmosis which drives water molecules through a selectively permeable membrane, resulting in purified water without the hardness and pollution found in rainwater and tap water. As such, reverse osmosis water has no hardness level and its pH is generally lower compared to normal fresh tap water.

One of the benefits of using purified water is that the limitations of your local water can be removed. Tap water may contain contaminants that can cause algae blooms in your aquarium. Therefore, by using reverse osmosis water, your aquarium will be free of any potential contaminants. Additionally, you can recreate a full range of aquatic habitats within your homes from scratch.

Limitations of Reverse Osmosis Water

One problem with reverse osmosis water in freshwater aquariums is that it may be too pure. It contains very few of the salts and minerals that your fish require for their environment.  The lack of minerals also creates a poor environment for plant growth, fishes’ survival and development of your aquarium’s microbial communities. Therefore, you should not use only reverse osmosis water. Instead, you explore ways to re-mineralize your reverse osmosis water. You should gradually experiment with different proportions to find out the water parameters you need.

Another problem with reverse osmosis water is that it has zero carbonate alkalinity (kH) and general hardness (gH). This means that reverse osmosis water has no buffering capacity and is very soft. Consequently, your aquarium will be vulnerable to huge swings in pH and other water parameters.

Most fish prefer a constant pH in the range of 6.4 to 7.8. They cannot live through large pH fluctuations, such as fluctuations of greater than 0.3 over a 24-hour period. Thus, huge changes in your aquarium’s pH levels will greatly compromise the health of your fish.

Therefore, you should always find ways to raise the kH and gH of your aquarium’s water. One way is through adding a commercial buffer or sea salts for a saltwater aquarium. However, when adding these products, you need to carefully monitor all the parameters to make sure you do not crash the aquarium.

Making Reverse Osmosis Water Suitable for your Aquarium

Remineralising agent

As pure water alone can create an unstable environment in your aquarium, you should explore ways to re-mineralise your aquarium. In pure water, there is zero carbonate hardness and a buffering capacity. This problem can be easily solved by using a re-mineralising buffer.

Re-mineralising buffers are available for both fresh and salt water use. This enables any fish keeper to produce the correct water parameters for every kind of fish. Many stores supply pre-mixed seawater that can be added straight to your aquarium complete with all the necessary minerals added.

Also, once you have decided to use reverse osmosis water, you should make it a point to constantly use it every time you top up or change the water as your pet fish will have already adjusted to these water conditions. It would be unwise to constantly use reverse osmosis water and suddenly switch to normal tap supply as this will place unnecessary stress on your fish as they will take time to readjust.


Overall, using reverse osmosis water in your aquarium has numerous benefits. With purified water, you can eliminate any nuisance minerals or salts from causing problems in your aquarium. With purified water, you can create any aquatic environment from scratch. This includes adding some crucial minerals to your reverse osmosis water is also key for the growth and survival of your fish.

You should also work towards establishing stable water parameters as they are key to a great set-up. If you subject your fish or corals to unstable conditions, they will likely experience at least some stress as they are getting used to the new water parameters.  In the case of severe fluctuations, you could experience losses with sensitive corals, invertebrates and fish.

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