How to Remineralise RO Water for Your Aquarium

Table of Contents

• Why RO Water Needs Remineralising
• Choosing the Right Remineraliser
• Dosing to Target GH and TDS
• Species-Specific Targets
• Mixing and Measuring
• Batch Preparation and Storage
• Common Mistakes to Avoid
• Singapore Context
• Frequently Asked Questions

Why RO Water Needs Remineralising

Reverse osmosis (RO) water is pure, almost distilled. An RO membrane strips out virtually everything from the source water: chlorine, chloramine, heavy metals, dissolved minerals, and organic compounds. What comes out the other side is water with a TDS (total dissolved solids) of 0 to 10 ppm, a GH (general hardness) of 0, and a KH (carbonate hardness) of 0.

This purity is exactly why aquarists use RO water in the first place. It provides a blank canvas that you can remineralise to precise specifications. But pure RO water on its own is dangerous for aquarium inhabitants.

The Dangers of Pure RO Water

• Zero GH: General hardness measures calcium and magnesium, minerals essential for fish osmoregulation, shrimp shell formation, and crustacean moulting. Without them, shrimp experience failed moults and fish suffer osmotic stress.
• Zero KH: Carbonate hardness acts as a pH buffer. Without KH, pH becomes unstable and can swing wildly with even minor changes in CO2 or organic acid levels. A sudden pH crash can be lethal.
• Osmotic imbalance: Fish and shrimp regulate their internal salt balance relative to the surrounding water. Water with zero dissolved minerals creates an osmotic gradient that forces the animal’s body to work harder, causing stress and eventually organ failure.

The solution is straightforward: add minerals back to the RO water before using it in your aquarium. This process is called remineralisation, and doing it correctly is essential for the health of your livestock.

Choosing the Right Remineraliser

Several commercial remineralisers are available, each formulated for specific purposes. Choosing the right one depends on what you are keeping.

For Caridina Shrimp

Use a GH-only remineraliser like SaltyShrimp GH+. Caridina shrimp (Crystal Red, Crystal Black, Taiwan Bee varieties) require soft, acidic water with GH of 4 to 6 and zero or near-zero KH. The acidic pH is maintained by buffering aquasoil, which absorbs KH. Adding a GH/KH+ product would work against the aquasoil’s buffering action.

For Neocaridina Shrimp and General Fish

Use a GH/KH+ remineraliser like SaltyShrimp GH/KH+. Neocaridina shrimp and most tropical fish prefer moderate hardness with some KH buffering to keep pH stable. A balanced GH/KH product provides both mineral content and pH stability.

For Planted Tanks

Seachem Equilibrium works well because its high potassium content benefits plant growth. Pair it with Alkaline Buffer if you need KH as well. This combination gives you control over GH and KH independently.

Dosing to Target GH and TDS

Each remineraliser has a specific dosing ratio, typically listed on the product label. The general approach is:

1. Fill a container with RO water (measure the volume precisely).
2. Add the remineraliser in small increments, stirring thoroughly between each addition.
3. Measure TDS with a TDS meter after each addition.
4. Stop when you reach your target TDS.
5. Verify GH with a liquid test kit to confirm the relationship between TDS and GH for your specific product.

Approximate Dosing Ratios

These are starting points. The exact relationship between the weight of remineraliser added and the resulting GH and TDS depends on the specific product batch and your measurement tools. Always verify with a GH test kit and TDS meter rather than relying solely on the label dosing.

Species-Specific Targets

Different species have different mineral requirements. Getting these right is especially critical for shrimp, which are far less tolerant of incorrect parameters than most fish.

For more detail on GH and KH, see our comprehensive GH and KH guide. For TDS-specific guidance, our TDS aquarium guide covers measuring techniques and interpretation in depth.

Mixing and Measuring

Equipment You Need

• TDS meter: An inexpensive, essential tool. Digital TDS pens cost under $20 SGD and provide instant readings. Calibrate monthly with a calibration solution for accuracy.
• GH test kit: Liquid drop test kits (e.g., API GH test) are more accurate than strip tests. Each drop that changes colour represents 1 degree of GH.
• KH test kit: Same format as the GH test. Essential if you are using a GH/KH+ product or separate KH buffer.
• Measuring spoon or scale: A small digital scale accurate to 0.1 grams ensures consistent dosing between batches. Measuring spoons work but are less precise.
• Mixing container: Food-grade bucket or jerry can. Dedicate this container to aquarium use only.

The Mixing Process

1. Fill your container with a known volume of RO water.
2. Measure your TDS baseline (should be 0 to 10 ppm for fresh RO water; if higher, your membrane may need replacing).
3. Add the remineraliser gradually, stirring well. SaltyShrimp products dissolve quickly; Seachem Equilibrium may take a few minutes.
4. Measure TDS after each addition. Approach your target slowly rather than overshooting.
5. Once you reach your target TDS, verify GH with a liquid test kit.
6. Record the exact amount of remineraliser you used for this volume so you can replicate it consistently.
7. Allow the remineralised water to reach tank temperature before adding it to the aquarium.

Batch Preparation and Storage

Preparing remineralised water in batches is more efficient and consistent than mixing small amounts for each water change. Here is how to set up a reliable batch process.

Storage Containers

Use food-grade HDPE containers (jerry cans, buckets with lids). Avoid containers that have held chemicals, soaps, or non-food substances. In Singapore, 20-litre food-grade jerry cans are available from hardware stores and online retailers for under $15 SGD.

Batch Process

1. Produce a batch of RO water (how much depends on your tank size and water change volume).
2. Store the RO water in your dedicated container.
3. Add the pre-measured amount of remineraliser and stir thoroughly.
4. Let the water sit for at least an hour to allow full dissolution and temperature equilibration.
5. Verify TDS before each use, as evaporation during storage can concentrate minerals slightly.

How Long Can Remineralised Water Be Stored?

Remineralised RO water can be stored in a sealed, food-grade container for up to two weeks without issues. Beyond that, bacterial growth may occur, especially in Singapore’s warm ambient conditions. If storing for longer periods, keep the container sealed and in a cool location. Always check TDS and give the water a sniff before use; if it smells off, discard it and prepare a fresh batch.

Common Mistakes to Avoid

• Adding remineraliser directly to the tank: Always pre-mix in a separate container. Adding concentrated remineraliser directly to the tank creates localised parameter spikes that can shock sensitive livestock, particularly shrimp.
• Using TDS alone without checking GH: TDS measures all dissolved solids, not just the minerals you added. If your RO water is not pure (membrane wearing out), the TDS reading may be misleading. Always cross-reference with a GH test.
• Inconsistent dosing between batches: Use a digital scale rather than estimating scoops. Shrimp, especially Caridina, are sensitive to even small parameter fluctuations. Consistency between batches is critical.
• Forgetting to temperature-match: Pouring cold remineralised water into a warm tank causes temperature shock. Prepare your water in advance and let it reach room temperature, or use an aquarium heater in the storage container.
• Using the wrong product for your species: A GH/KH+ product in a Caridina tank with buffering aquasoil causes the soil to exhaust its buffering capacity faster, leading to eventual pH instability. Match the product to the species.

Singapore Context

Singapore’s PUB tap water has a TDS of approximately 50 to 120 ppm, GH of 1 to 4, and KH of 1 to 3, which is relatively soft by global standards. For many fish species, this water is perfectly adequate after dechlorination, and an RO system may be unnecessary. However, for Caridina shrimp breeders who need precise, consistent parameters, RO water with controlled remineralisation is the standard approach.

The main reasons Singapore hobbyists use RO water include:

• Breeding Caridina shrimp that require very soft, acidic water with precisely controlled GH
• Keeping discus or other soft-water species at specific low TDS values
• Eliminating chloramine and other PUB treatment chemicals at the source (though a good dechlorinator achieves this without RO)
• Ensuring parameter consistency between water changes, as PUB water parameters can fluctuate slightly between seasons

For more on whether you actually need an RO system, read our complete guide to RO water for aquariums.

Frequently Asked Questions

Can I use tap water instead of remineralised RO water?

For most tropical community fish and Neocaridina shrimp, Singapore tap water (after dechlorination) is perfectly acceptable. RO remineralisation is primarily necessary for Caridina shrimp, discus, and other species that require parameters different from what tap water provides. If your tap water parameters are suitable for your livestock, there is no need for the added complexity and cost of an RO system.

What happens if I add too much remineraliser?

If you overshoot your target TDS, the simplest fix is to dilute with more RO water. Measure TDS, calculate how much RO water you need to add to reach the correct concentration, and mix thoroughly. This is much easier than trying to remove minerals from overly mineralised water, which is why approaching the target gradually is important.

Do I need to remineralise water for top-ups, or only for water changes?

For top-ups to replace evaporated water, use pure RO water without remineraliser. Evaporation removes only water, not minerals, so the minerals remain in the tank and become more concentrated as water evaporates. Adding remineralised water as a top-up gradually increases the mineral concentration over time, pushing TDS higher and higher. Only add remineralised water during actual water changes, where you are removing and replacing water that contains minerals.

How often should I test my remineralised water?

Test every batch before adding it to the tank. Once you have established a consistent dosing routine and confirmed it produces the correct parameters over several batches, you can rely on TDS as a quick check and do full GH/KH testing monthly. Always test if you open a new container of remineraliser, as different production batches may have slight variations.

Get Your Parameters Right

Remineralising RO water is a skill that every serious shrimp keeper and planted tank hobbyist in Singapore should master. With the right product, consistent technique, and accurate measuring tools, you can provide your livestock with precisely tailored water conditions that promote health, breeding, and vibrant colouration. Visit Gensou at 5 Everton Park, Singapore for remineraliser products, TDS meters, and expert advice on dialling in your water parameters. With over 20 years of experience, we have helped countless hobbyists get their water chemistry right from day one.

Related Reading

• How to Remineralise Tap Water for Shrimp
• Aquarium Cloudy After Water Change: Why and How to Fix It
• How Often to Change Aquarium Water: Finding Your Schedule
• Water Change Temperature Matching: Why It Matters
• Aquarium Water Conditioners Explained: Prime, Safe and Beyond