We get asked regularly about reverse osmosis water and whether it is better for fish tanks or not.

Reverse Osmosis, commonly referred to as RO, is a process of filtration where the membrane will allow the water molecules through, but will block larger molecules such as minerals and other contaminants.

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 growth 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 (parts per million) or higher which fish and corals are not used to living with these levels of TDS in the wild. With RO water this can be as low as 2ppm.

Furthermore, the quality of your tap water may vary throughout the year.  This would make it very hard to be consistent with your water quality and these changing water conditions can have adverse effects on sensitive fish and corals.

Choose Reverse Osmosis Water

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.

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. You may need to explore ways to re-mineralize your reverse osmosis water. This problem can be easily solved by using a re-mineralising buffer.

You should gradually experiment with different proportions to find out the water parameters you need ensuring that the pH remains constant and does not suffer large fluctuations as this will greatly compromise the health of your fish.

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 which 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.