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Is RO Water Safe for Freshwater Fish

Reverse osmosis water (RO) is great for removing impurities and reducing risk of contamination. It contains no lead and no parasites. It is great for humans because it has been proven to reduce the risk of high blood pressure and kidney or liver problems associated with tap water. Right now, you may be wondering to yourself, ‘is RO water safe for fish?’

Is RO water safe for freshwater fish? RO water is a good choice for your aquarium provided that you also have some mineral content present. Without any significant hardness to it, RO water is simply too pure for aquarium use and must have some beneficial salts and minerals added to it otherwise your fish will not survive. Mixing RO water with tap water is the best solution.

Now that you know RO water is okay for freshwater fish, let’s explore this topic in more detail below. We will look at which types of water are best for freshwater fish, how long (if possible) can fish survive in RO water, how to dechlorinate water in an aquarium and whether or not betta and other tropical fish can live in a reverse osmosis or pure water environment.

So, if you’re ready to learn all you need to know about RO water use in aquariums, then let’s get stared!

Can Fish Survive in RO Water?

Freshwater fish cannot survive in an aquarium with RO water only. It is best to mix reverse osmosis water with tap water as the RO component will soften the water and lower the PH. If you decide to use 100% RO water than you will have to ‘harden’ it by adding essential salts and minerals in the correct amounts required to ensure the appropriate parameters (or water conditions) are met.

Which Water is Safe for Freshwater Fish?

Water parameters are the key to a healthy tank. Minerals are necessary for freshwater fish in an aquarium and plants will act as a ‘buffer’ for pH levels. Distilled water (along with proper remineralization) is one of the best choices you can make for the safety and health of your fish. Pure distilled water, however, can be fatal for fish and significantly alters pH levels.

Fish-keepers can make tap water safe for aquariums by pre-treating it with a liquid water conditioner purchased either online or at a pet store. A product such as StressCoat instantly detoxifies both the chlorine and chloramine in the water and neutralizes any heavy metals present. It is available for purchase through Amazon and is reasonably priced. There are other brands available as well that are decent products if you look at StressCoat and feel it’s not for you.

Remember, partial water changes are necessary in preventing the overgrowth of algae and keeping both the water and fish safe. They also help lower nitrite and phosphate levels and manage waste deposits. Trace minerals and elements in the water are vital to the health of the fish and plants in the aquarium and maintain the stability of the water chemistry, which is why only a part (not all) of the water is removed and replaced.

Lately I have really been on a water change kick when telling people the best thing they can do for their aquarium. It’s just so extremely important to do.

Is Spring Water Good for Fish?

Spring or ‘bottled’ water alone is not good for freshwater fish in an aquarium. It does not have the necessary minerals to regulate the pH levels. Spring water, as opposed to distilled or RO water, can vary in terms of composition (mineral and salt content) creating an unstable environment in a tank. Therefore, spring water alone is not recommended. I have never used spring water in my tanks.

Is Rain Water Good for Fish?

Rain water is basically soft, acidic water and can be used in a freshwater tank as long as you mix 10% tap water as well. Some species of fish, such as bettas, tetras and rasboras can survive in pure or almost pure rain water environments. However, for most other fish, you would have to treat and buffer rainwater in order to make it suitable. On a side note if you have Betta fish make sure to collect rain water periodically as a treat for the fish.

Is Reverse Osmosis Water Safe for Betta Fish?

Speaking of betta fish, you can use RO water for them, however, it may cause bacterial infections in the fish if the water is very soft and the pH levels are too low. Bettas will thrive in an environment with a neutral pH somewhere around 7.0. Most tap water pH levels are between 6.5 and 7.5. I see posts online where a Betta owner is asking for help with their fish because they appear to have some sort of infection. If the water in the tank is too pure it could be the cause.

Can You Use RO Water for Tropical Fish?

You can use RO water in a tropical fish tank as long as you mix in some tap water as well to create the desired pH level. For example, if the hardness (or gH) of the water is 20 and what you need in your tank is a gH of 5, make the solution 80% to 90% RO water and 10% to 20% tap water to start and adjust the amounts accordingly. Test the water with pH strips to ensure it is within the proper parameters.

Reverse osmosis water is especially good to use when just starting a freshwater aquarium. It keeps the growth of algae down while you are preparing the tank for fish and other aquatic life. Re-mineralizing it in order to create the proper water parameters is then required. Or just perform water changes using treated tap water.

How Do You Dechlorinate Water for Fish?

To dechlorinate water in an aquarium often requires the use of an additive or dechlorinating agent. However, investing in a good filtration system will remove chlorine at the source and save you both time and money in the long run! The two most common ways of dechlorinating water in a tank include:

Add a Dechlorinating Agent to the Water

Begin by purchasing a dechlorinating agent online or at your local pet shop. Simply unscrew the lid from the bottle and tip it upside into the tank, allowing the proper amount to drip slowly out. After adding the product, the water will be ready for immediate use. If you happen to be using a biological filter in the tank then choose a product that doesn’t contain an ammonia remover. This could cause problems with your filter.

One product to consider is Seachem Prime Fresh and Saltwater Conditioner. It is readily available online at Amazon and best of all can be delivered right to your front door! This product is affordable. It is safe for all forms of aquatic life and helps prevent against fish gill damage. Go online and check it out today!

Aerate the Tank Using an Air Pump

While it is recommended to always dechlorinate the aquarium water before adding fish to it, aerating the water will also help remove unwanted chlorine. Simply purchase the appropriate pump for your tank size which you can find at online retailers or at your local pet shop. Most tanks typically require an air pump to circulate the water anyway. Its ability to removing harmful chlorine is just an added bonus and watching bubbles in an aquarium is just a lot of fun.


In conclusion, RO water is safe for freshwater fish in a tank as long as there is some mineral content present. Without any significant hardness (or PH level) present, RO water is simply too pure for freshwater aquarium use and must have some beneficial salts and minerals added to it or your fish will not survive. Mixing RO water with tap water is the best solution. Doing partial water changes as well will help keep the water safe for fish. Good luck, hobbyists and happy fish keeping!

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