How to Remineralize RO Water for Freshwater Aquariums
If you want more control over your water parameters, then starting with pure RO/DI water is the way to go. Reverse osmosis and deionization systems will remove chemicals, organics, & other dissolved solids from tap water, leaving you with a perfect blank slate for builiding your ideal aquarium water conditions.
The process of adding back key mineral nutrients to this pure water is called remineralization, and it is a crucial step for the health of your aquarium fish, plants & invertebrates.
In this guide, we'll explain the importance of remineralization, when and how to do it, and the relationships between crucial water parameters like GH, KH, and TDS.
The Importance of Minerals in Aquarium Water
Water in lakes, rivers, & streams naturally contains dissolved minerals such as calcium, magnesium, potassium, and sodium.
Fish, aquatic plants, and invertebrates rely on these minerals for a variety of biological processes:
- Calcium is important for skeletal development and other physiological processes.
- Magnesium plays a role in enzyme activity and is an important nutrient for plants.
- Potassium is essential to numerous cellular processes and is particularly important for aquatic plants.
- Sodium contributes to proper nerve and muscle function.
Because the exact mineral balance of the water will vary by geographic location, different species of fish and plants will often have different mineral needs. This is why it's important to research the species you wish to keep prior to adding them to your tank. When it comes to remineralization, the goal is always to establish water chemistry that's appropriate for the species you're keeping.
What Are GH, KH, and TDS?
Understanding GH, KH, and TDS is important when working with remineralized water.
GH - General Hardness
GH measures the concentration of dissolved calcium and magnesium ions in water.
The ideal GH will depend on the species you keep. For example, many freshwater shrimps and snails require GH levels between 6-10 dGH for proper growth and shell development, whereas soft water species like Cardinal Tetras prefer setups under 4 dGH.
KH - Carbonate Hardness
KH measures carbonate and bicarbonate compounds in the water, which influences the aquarium's buffering capacity (ability to maintain stable pH).
The appropriate KH will also depend on the species and the type of aquarium you're maintaining, but generally a minimum level of ~2-3 dKH is recommended to prevent pH swings.
TDS - Total Dissolved Solids
TDS meaures the total concentration of dissolved substances in water.
Although TDS can be a useful metric for checking RO/DI water purity and monitoring changes in your aquarium over time, it is not a useful measurement for remineralization.
A TDS reading can only tell you the total amount of dissolved substances, not exactly what those dissolved substances are. Because of this, two aquariums could have the same TDS reading while having very different concentrations of calcium, magnesium, potassium, or other substances.
Therefore, TDS should not be used by itself to determine whether your water is appropriate for a particular species.
GH, KH, and other relevant water tests provide more useful information about the actual composition of your aquarium water.
When Should You Remineralize Water?
Using RO or RO/DI Water
If you're using RO/DI water for a freshwater aquarium, you'll need to remineralize it before using it since RO/DI filtration removes minerals along with unwanted contaminants.
Using Distilled Water
Distilled water is another type of purified water that contains very few dissolved minerals.
If you're using distilled water for aquarium water changes, it may also need to be remineralized before being added.
Using Very Soft Tap Water
If your tap water is naturally very soft, it may contain insufficient concentrations of certain minerals for the species you're keeping. Testing your source water can help determine whether additional minerals are necessary.
How to Remineralize RO Water for a Freshwater Aquarium
The exact process will vary depending on the remineralizer you're using, but the basic steps are straightforward.
Step 1: Prepare Your Source Water
Start by producing your RO/DI, distilled, or dechlorinated tap water in a clean container.
If you're using an RO/DI system, check the TDS of the product water. A very low TDS reading indicates that the system is effectively removing dissolved substances.
Step 2: Choose an Aquarium Remineralizer
Choose a remineralizer based on the requirements of your aquarium.
Different products are formulated to affect different parameters. Some primarily increase GH, while others increase both GH and KH. Some are specifically formulated for certain types of freshwater fish, shrimp, or planted aquariums.
Always read the manufacturer's instructions and research the water requirements of your livestock before choosing a product.
Step 3: Add the Remineralizer to your Step 1 Container
Always mix remineralizer into your replacement water rather than adding concentrated remineralizer directly to the aquarium.
Measure the appropriate amount according to the manufacturer's instructions and mix it thoroughly into the water.
Preparing the water separately allows you to test and adjust it before exposing your fish, plants, or invertebrates to a change in water chemistry.
Step 4: Test GH & KH
Once the remineralizer has dissolved, test the water.
Depending on your aquarium, useful parameters to monitor may include:
- GH
- KH
- TDS
- pH
Your target values should be based on the requirements of your particular species and the current parameters of your aquarium water. When performing a large water change, it's important to match the existing aquarium parameters with the new water as closely as possible.
Step 5: Adjust Gradually
If your water needs adjustment, make changes gradually over time.
Rapid changes in water chemistry can stress fish and invertebrates. This is especially important when working with sensitive species or making large water changes.
Why Use RO/DI Water Instead of Tap Water?
Tap water can contain a wide variety of dissolved minerals and other substances, including unwanted chemicals or heavy metals. Its composition can also vary depending on your local water supply and routine city maintenance. RO/DI water removes most of that variability and gives you a blank slate so that you can add back the minerals you want in a controlled manner.
This can be particularly helpful for:
- Sensitive freshwater species
- Shrimp aquariums
- Planted aquariums
- Breeding projects
- Aquariums requiring very soft water
- Aquarists whose tap water chemistry isn't suitable for their livestock
What Are the Ideal Water Parameters?
There isn't a single GH, KH, or TDS value that's ideal for every freshwater aquarium.
Your target should be based on the natural environment and specific requirements of the species you're keeping.
Before adjusting your water chemistry, research:
- The species' natural habitat
- Recommended GH
- Recommended KH
- Recommended pH
- Temperature requirements
- Other known water chemistry requirements
When keeping multiple species together, make sure their water chemistry requirements are compatible.
Stability is Key
It's easy to get caught up in trying to hit a specific number on a test kit; However, stability is extremely important in an aquarium.
If your livestock is healthy and your water parameters are appropriate and stable, making unnecessary adjustments can potentially create more problems than it solves.
Make changes only when they're needed, and make them gradually.
Breeding Aquariums
Remineralization can be particularly useful when breeding freshwater fish and invertebrates.
Some species reproduce most successfully under very specific water conditions. Starting with RO/DI water allows an aquarist to establish a more controlled GH and KH instead of relying entirely on the mineral composition of their tap water.
If you're setting up a breeding aquarium, research the water parameters recommended for the specific species rather than relying on a generic remineralization recipe.
Key Takeaways
- RO/DI and distilled water remove minerals along with unwanted contaminants.
- Freshwater aquariums often need remineralized RO/DI water to provide essential minerals.
- GH primarily reflects calcium and magnesium concentrations.
- KH measures carbonate and bicarbonate compounds that contribute to buffering capacity.
- TDS measures total dissolved solids but doesn't tell you which substances are present.
- The ideal water chemistry depends on the species you're keeping.
- Always mix remineralizer into replacement water separately before adding it to your aquarium.
- Make changes gradually and prioritize stable water chemistry.
- Use species-specific GH, KH, and other water chemistry recommendations rather than chasing a universal number.
Frequently Asked Questions
Does RO water need to be remineralized for a freshwater aquarium?
Yes, RO water contains very few dissolved minerals, so it generally needs to be remineralized before being used as the primary water source for a freshwater aquarium. The appropriate mineral levels depend on the species you're keeping.
Does RO/DI water have minerals?
RO/DI water has very few dissolved minerals because reverse osmosis and deionization are designed to remove dissolved substances. High-quality RO/DI water can have a TDS reading near 0 ppm.
What is the difference between GH and KH?
GH measures the concentration of calcium and magnesium ions, while KH measures carbonate and bicarbonate compounds that contribute to the water's pH buffering capacity.
Can I use TDS to determine how much remineralizer to add?
TDS can be useful for monitoring changes in water over time, but it doesn't identify which substances are contributing to the reading. For that reason, TDS should not be used as the only measurement when determining whether water is appropriately remineralized.
Can I remineralize distilled water?
Yes. Like RO/DI water, distilled water contains very few dissolved minerals and can be remineralized before being used for a freshwater aquarium.
Should I remineralize tap water?
It depends on your tap water and the species you're keeping. If your tap water is already sufficiently mineralized, additional remineralization may not be necessary. Test your water first.
Can remineralization raise pH?
Some remineralizers can affect KH and therefore influence the aquarium's buffering capacity and pH. The effect depends on the specific product and its formulation. Always check the manufacturer's instructions and test your water after mixing.
How quickly should I change aquarium water chemistry?
Water chemistry should be changed gradually rather than rapidly. Sudden changes in GH, KH, pH, or other parameters can stress fish and invertebrates. If there's a large gap between your current and desired water parameters, make small adjustments over the course of weeks.
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