How to Remineralize RO Water for Aquarium?

Community fish have this impressive ability to adapt to a variety of water conditions and other environmental factors.

However, most aquarists still want to provide their beloved fish with the best kind of water.

RO water (Reverse Osmosis water) is free from all the minerals and compounds that could potentially harm their guppies, betas, and even turtles, but it’s also not good to have water that has no minerals.

You need some important minerals and water conditions for your fish tank.

The good news is that you can easily add a few things to RO water and make it a perfect choice for your fish tank.

This blog will serve as a comprehensive guide to remineralizing and using RO water for your aquarium.

We will talk about the process of adding useful minerals to the water, how reverse osmosis works, and even why RO water is best suited for your water tanks.

How to Remineralize RO Water for Aquarium?

If you’re using RO water for your aquarium, there is an extra step you’ll have to take. RO water undergoes purification to the point where it contains no minerals.

So, you will need to re-introduce certain compounds.

These compounds offer several health benefits to your fish and help maintain ideal living conditions for the aquarium-life.

Maintaining the Right Conditions

Most of your fish will survive best at a relatively neutral pH of around 7-7.5.

Here are some other water parameters that you will need to take into consideration.

  • Total Dissolved Solids – 390 ppm
  • Carbonate Hardness – 8 to 9 kH
  • General Hardness – 10 GH

Best Remineralization Practices

Remineralization depends on the percentage of RO water you are using and the parameters you wish to maintain.

This simple recipe works great to achieve those water conditions per 5 gallons of tank water (considering it is 100% RO water).

  • 5 mL of water conditioner (approximately one cap full)
  • One teaspoon baking soda
  • ¼ teaspoon acid buffer

Before starting the remineralization process, you will need to rinse and wash out your tank thoroughly.

Give it a nice spin with tap water and leave it out to dry before adding it to the remineralized RO water.

If you don’t have access to the chemicals mentioned above, you can even replace them with some common ingredients.

A mixture of calcium chloride, Epsom salt, and baking soda does wonders to remineralize RO water for your fish tank.

For a more streamlined approach, we recommend installing remineralizing cartridges to your tank.

These cartridges are efficient in re-establishing your tank water’s alkalinity after the RO process has filtered out all the minerals.

It works best for aquariums with higher pH requirements, e.g., if you’re keeping mollies, swordtails, or rainbowfish.

Lastly, if you keep a freshwater tank, the best way to restore its mineral content is by using Seachem Equilibrium.

It functions to achieve the optimal mineral and electrolyte balance for your aquarium and maintain a stable carbonate hardness.

Is it Important to Remineralize RO Water?

RO water is virtually free of any chemicals and minerals.

The reverse osmosis process also neutralizes the water’s pH and ensures that there is no water hardness.

RO water gives you a completely blank slate to work with.

Quite a few tropical and community fish have specific water requirements. Some require alkaline waters, and others need significant salt content.

While your ordinary tap water fails to meet these requirements, the RO water is also too ‘blank’ for them. That is where remineralization comes in.

By adding certain compounds and chemicals, like baking soda, you prime the water to best suit your fish’s needs.

It is the ideal solution to create a ‘custom-made’ living environment for your precious aquatic life.

Why Use RO Water for your Aquarium?

The process of remineralization and selecting specific buffers may sound daunting.

But don’t let that put you off from using RO water for your aquariums.

The reverse osmosis process ensures that your water is clean and pure.

It removes all harsh chlorides, benzene, arsenic, pesticides, toxins, and other organic and even inorganic substances.

Thus, you create a neutral water environment. RO water also eliminates:

Water Hardness

Do you often find crusty build-up on your faucets and sinks? That’s because your water pipes have hard water flowing in them.

When the water has high amounts of calcium and magnesium, it is said to become ‘hard’ and stain your bathtubs.

Water hardness isn’t a health concern among humans; hence it isn’t regulated by the local water department. However, it can be disastrous for our fish.

Hard water dramatically limits the variety of fish you can introduce to your water tank.

Although some community fish do live well in these hard water conditions, like guppies and archers, most won’t farewell.

Freshwater and tropical fish have very different temperaments. They are unable to live without a proper supply of soft water.

The reverse osmosis system makes it possible for you to offer these tropical fish their optimal living conditions.

It makes the water neutral and removes all water hardness. This way, you can control the exact degrees of hardness your fish need.


When nitrogen components combine with oxygen, we get nitrates. Nitrates are generally found in large quantities in our water supply.

Although they have no significant effect on human health, these nitrates can disturb marine life immensely.

Your fish are quite sensitive to nitrates. Furthermore, as the nitrate content of your water increases, it will promote algal growth, which will quickly spread throughout your aquarium.

As more nitrates are formed, the oxygen in your water tank will be depleted.

These suppressed water oxygen levels will put your fish under stress and negatively affect their general well-being.

If these nitrate levels cross 100 ppm, even the most resilient of your aquarium fish are likely to struggle and show signs of poor health.

This can be particularly disastrous to any young fish and delicate fry.

Based on the water environment of your aquarium, your nitrate levels should be highly controlled.

  • Freshwater aquariums – up to 40 ppm
  • Saltwater aquariums – 10to 40 ppm
  • Reef aquariums – lower than 5 ppm

So, if your aquarium tank has invertebrates, corals, or sea cucumbers, even the slightest rise in nitrate levels will damage them beyond repair.

Your Sea Apples can’t handle exposure to these nitrates either.

The main issue is that fish produce nitrates as a waste byproduct. So naturally, its levels keep on increasing.

This is where RO water shows its users the most.

When your water tank starts with almost no nitrates, this waste release doesn’t immediately result in high levels.

Instead, the nitrates slowly build up and will be removed once again as you change the water in your tank.

To further clear your water of nitrates, you can also add mangrove plants and even nitrifying bacteria that live on a rock.


Silicates are some of the most common contaminants found in wells and tap water.

Also, aquariums with sandy substrates constantly leach out silicates into your tank water.

These contaminants can take over your tank in a matter of weeks and start producing silica algae.

Silica algae, also called gravel algae, has rampant growth and spreads across rock beds. You will find it coating your aquarium’s floor with a brown film in no time.

Moreover, this film stops light from entering the aquarium and reduces oxygen levels.

If your aquarium has coral, these algae will be your worst enemy. Not only do they deplete oxygen, but some even have a carnivorous effect on corals.

They start by populating the base of your coral and slowly reaching upwards to consume it. The coral’s skeleton is damaged, and it starts to decay.

The best way to get rid of silicates present in your water is by using reserve osmosis.

As these contaminants cant cross the RO membrane barrier, they won’t be able to get to your coral and gobble them up.


If you notice your tank water turning green and murky despite regular cleaning, that is a sign of high phosphate content.

While phosphates won’t directly harm your fish’s health, they do promote the growth of algae.

This green algal growth will quickly take over your tank and cause a massive drop in oxygen levels.

Regular tap water generally has at least 1 ppm of phosphate, with the highest levels reaching 10 ppm.

Furthermore, fish waste, uneaten food, and decaying plants also increase the phosphates in your tank.

Your coral community can’t survive more than 0.2 ppm of phosphate.

Since there is no way of filtering phosphates out once they are in the tank, it’s best to start with RO water.

Giving your fish the lowest phosphate levels possible will ensure algal growth is minimal, and your tanks don’t turn into algae fields.

A Balanced Aquatic Environment

With the neutral environment of RO water, you can easily design the best living conditions to see your fish grow and thrive.

RO water also lets you control the pH levels, water hardness, salt content, and mineral balance.

It is the best way to offer your fish a stable water tank, regardless of how good or bad the water running in your pipes may be.

Most seasoned aquarists can tell you tales of how the smallest imbalances in water tank conditions devastated their marine life.

Fish are sensitive creatures that require unique combinations of minerals and pH to live happy and healthy lives.

Even slight diversions from these parameters can wreak havoc on your aquarium.

For example, your angelfish and discus demand very soft waters with a neutral pH. If you live in a hard water territory or try to use well water for this tank, your fish won’t survive.

On the other hand, Livebearers like platies and guppies, as well as archers and Monos, need very hard water with a pH of 8 and higher.

These resemble the natural living conditions of these fish and thus work best for them.

Final Words

Regardless of what water parameters you need, it is always best to use the ‘blank’ RO water. You can then make the required adjustments easily.

When you start with 100% pure water, the results of adding acidic buffers or mineralized solutions become predictable and can tweak them to your fish’s liking.

No trial and error is needed!

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