Can You Use RO Water for Aquarium? (But Why)

With the world evolving around us we cannot overlook the fact that it covers every aspect of our everyday lives – even the main source of life on our planet. This time let’s tackle the most advanced water purification process of reverse osmosis.

During this process, your water is filtered to remove ions, all unwanted particles, and even molecules whilst remaining at the ideal pH level of 6.5.

It is safe to say that most of us understand the advantages of RO water for the human body but how about the ones that cannot breathe without the surroundings of the water?

This article will answer everything you wondered about before using RO water for your aquarium. Therefore, keep on reading and find out the safety of the RO water for your fish tank, the reasons for it, and its advantage compared to regular tap water. 

Is RO Water Safe for Aquariums?

RO water is safe for your aquarium as it provides a neutral environment that you can customize to the needed conditions.

Because the purification process during which reverse osmosis water is produced uses semipermeable membranes, it assures the elimination of all the nitrates, heavy metals, salts, and minerals.

The latter, as aforementioned, creates a blank page for you to craft the environment of your fish tank. This indeed requires knowledge, but the result of having the ideal conditions for your water pets and plants makes the RO a complete winner amongst all the filtered waters.

Why Should You Use Reverse Osmosis for Your Fish Tank?

If you are still in doubt about whether to use RO water for your fish tank, here are other important measures as to why RO water is perfectly suitable for your aquarium:

  • No chemicals or biological bacteria

When the RO water is produced, an applied pressure is used to overcome osmotic pressure. The solute is retained on the pressurized side of the membrane, and the pure solvent is allowed to pass to the other side.

To be “selective,” this membrane does not allow large molecules or ions through the pores) but allows smaller components of the solution (such as solvent molecules, i.e., water, H2O) to pass freely. Therefore, leaving no chance for the negative particles to align in the fluid.

  • Neutral pH

When producing reverse osmosis, water minerals like calcium and magnesium are removed, making the water become acidic. Why is that? The weaker are acids, the easier they transfer hydrogen ions to the solution. Whereas alkanite liquids work the opposite, meaning that the more alkanite something is the easier it will accept those hydrogen ions. When making RO water, alkaline mineral atoms are filtered, making RO water almost neutral pH.

  • No water hardness

Due to the method the minerals and particles removed, RO water’s general hardness is 0 ° making it the softest water option.

  • Perfect for reef aquariums

Other water options (e.g., tap water) might contain excessive chlorine, chloramines, copper, nitrates, nitrites, phosphates, and silicates which can irritate sensitive reef environments. Therefore, RO water is free of all the components and perfectly duplicates natural reef conditions.

  • No algae growth

Nitrogen and phosphates that can be found in ordinary tap water might influence unwanted algae growth. With RO water, there will be no more unwanted algae as it does not contain any of the abovementioned contaminants.

  • Suitable for tropical fish breeding

Although tropical fish are able to survive in a suitably treated tap water-filled aquarium, it is still impossible to breed in such conditions. Hence, many professionals use RO water instead as its softness replicates the majority of the tropical water bodies making it the perfect environment both to live and breed.

Why Reverse Osmosis Water is Better than Tap Water?

Tap water is normally full of various elements, including heavy metals, minerals, and salts. When some of these components are crucial for both humans and fish, most might harm sensitive habitats of the water life.

RO water is unique as it is completely pure and has no aforementioned elements at all. Even though it completely eliminates general hardness making the water ideally soft for the fish and water plants, some minerals and chemicals are still needed for the fish to live.

This means that you will have to adjust the fish tank accordingly by adding some of the elements eliminated in the RO process.

To draw a clearer picture, below, we examine the elements found in RO water and tap water which you can then compare to the ideal water component levels accordingly to your fish tank’s specialty.

  • Depending on your fish type, the chart below shows the ideal water component levels and conditions for your fish tank.
 CommunityAfrican CichlidsPlants & Soft Water FishDiscusBrackish
pH7.0 – 7.88.0 – 9.06.5 – 7.46.5 – 7.07.8 – 8.4
KG4o – 8o10o – 25o1o – 4o1o – 4o8o – 18o
GH4o – 10o15o – 25o3o – 6o3o – 6o10o – 20o
Nitrate< 50 ppm< 50 ppm< 30 ppm< 10 ppm< 30 ppm
Phosphate< 0.5 ppm< 0.5 ppm< 0.05 ppm< 0.05 ppm< 0.5 ppm
  • The following table portrays the levels of the above-shown components as found in RO water and ordinary tap water. Compare this data to the ideal measures and create a perfect aquarium environment for your fish.
ComponentRO waterTap water
pH6.56.5 – 9.5
KG0 °depends on the country
GH0 °8°- 30 °
Ammonia0.00.25 – 32.5
Nitrate< 0 ppm<10 ppm
Phosphate< 0ppm<0.05 ppm

Is Reverse Osmosis the Best Water for your Fish Tank?

A straight answer to whether RO water is best to use for your aquarium is yes. However, there are a lot of things to consider before preparing your fish tank with RO water. Because of its purity, reverse osmosis water is eliminated from harmful bacteria, chemicals, and many elements that are crucial for your fish to live.

In this way, RO water is the best simply because how it is suitable for all water creatures, including reefs, tropical fish, plants, and many more. That is only if customized accordingly to every species, whether they live in saltwater or freshwater.

Some also add that a mix of RO water and tap water is a perfect solution for those who want an easier way out. Yet keep in mind that tap water might contain plenty of harmful elements; thus, it requires careful preparation beforehand.

How Do you Buffer RO Water in an Aquarium?

When preparing your fish tank with RO water, first and foremost, you have to evaluate all the living fish and plants in your aquarium. That is evaluate their breed and their natural living conditions. With that data search for the levels of each mineral, pH etc. I have already provided a table of the exact levels of the elements in RO water, therefore you have to adjust the measures to a needed constant. E.g. for your freshwater inhabitants you can lower pH levels by adding CO2 or acid buffers, likewise, use baking soda or other specialized products to lower KH levels

Check out this video for a more in depth examples:

Best Reverse Osmosis Systems to produce water for your aquarium

When considering buying a reverse osmosis system for your aquarium search for the capacity and power that suits you the best. Therefore, these are our 5 best options:

  1. Home Master TMHP HydroPerfection RO System – has the highest reviews ranking, has 9 filters and is able to work even with private wells, has UV light module.
  1. Home Master TMAFC-ERP Artesian Reverse Osmosis – patented with a “Full Contact’ remineralization module, which means that water is remineralized in two stages leaving it with a perfectly balanced pH.
  1. iSpring RCC7AK Under Sink Reverse Osmosis System – easy to maintain and clean and is sold with a 1-year money-back guarantee and a 1-year manufacturer warranty.
  1. Express Water RO5DX 5 Stage RO System – easy to install and adjust to one’s needs by adding a permeate pump or extra purification modules.
  1. APEC ROES-50 5-Stage RO System – perfect price and quality ratio, suitable for a household with lower water usage and generally clean water.

Find more about these products and their prices on Best Reverse Osmosis Systems page.

To Conclude

Reverse osmosis water is the purest water possible as it has all the harmful elements eliminated, yet the good ones are as well. RO water is perfect for your fish tank but only when adjusted to the right pH, mineral, and hardness levels. Consider using RO water for your aquarium and enjoy a clean and ideal environment for your fish and plants whether the reef or tropical.


This is my blog about the ways and solutions that can help you improve your health by taking more value from drinking water. As improving health means a lot to me, I decided to create the Water On Top project with the purpose to reveal the benefits of water, the finest products to take our daily water to another level, and much more great stuff about water that I believe is on top of our nutrition.

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