RO Water Mixing Calculator
Calculate the exact ratio of RO to tap water needed to hit your target GH and KH — includes remineralisation quantities for Seachem Equilibrium and baking soda when mixing alone can't reach your target.
Calculator
Discus / soft water
Very soft, low-KH water for sensitive soft-water species
GH 2 · KH 1
Soft planted tank
Ideal for CO2-injected planted tanks and soft-water species
GH 4 · KH 2
General tropical
Suitable for most community fish and moderate planted tanks
GH 8 · KH 4
Hard water species
Cichlids, livebearers, and other hard-water species
GH 12 · KH 6
Select a preset to fill in target GH and KH, then adjust as needed
Target parameters
General hardness — calcium & magnesium
Carbonate hardness — buffers pH
Tap water readings
Test your cold tap water directly — values vary by region and season
Volume unit
Enter your values above to see results
For dialling in your water profile
Essential for soft-water and shrimp setups.
Essential for dialling in your water profile — test tap and RO water before mixing
Instant dissolved solids reading — verifies your blended mix and catches changes after water changes
Raises GH without affecting KH — the standard for remineralising RO water in planted tanks
I’ve used everything listed in my own tanks. If you buy through a link, I earn a small commission — at no extra cost to you, and it never affects what I recommend.
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Frequently Asked Questions
- Why do planted tank hobbyists use RO water?
- RO (reverse osmosis) water removes virtually all dissolved minerals, giving you a blank canvas to work with. This is valuable because tap water varies significantly by region — some areas have very hard water (high GH and KH) that is unsuitable for soft-water species like discus, cardinal tetras, and many aquatic plants from Amazonian or Southeast Asian habitats. By blending RO water with tap water, you can dial in any target GH and KH precisely, regardless of what your tap water provides. It also removes chlorine, chloramine, heavy metals, and other tap water additives that can stress fish and shrimp.
- What GH and KH should a planted aquarium have?
- For most planted aquariums with a mix of common tropical fish, a GH of 6–10 dGH and KH of 3–5 dKH works well. Soft-water species like discus, apistogramma, and many wild-caught tetras prefer GH 2–6 and KH 1–3. High-tech planted tanks with CO2 injection can tolerate lower KH (1–3 dKH) since CO2 manages pH rather than carbonate buffering. Hard water species like cichlids, livebearers, and goldfish thrive at GH 10–20 and KH 6–12. The most important thing is stability — large swings in GH or KH stress fish more than being slightly outside the ideal range.
- How do I remineralise RO water?
- The most common approach is to use Seachem Equilibrium to raise GH and sodium bicarbonate (baking soda, NaHCO3) to raise KH. Equilibrium contains calcium, magnesium, and potassium in a ratio that mimics natural soft water and is particularly suited to planted tanks. Baking soda raises carbonate hardness (KH) and therefore pH buffering capacity without significantly affecting GH. Dose both into the RO water before mixing with tap water. Commercial products like Seachem Replenish or Brightwell Aquatics Reef Code A can also remineralise RO water and may be more convenient for smaller tanks.
- Does RO water affect CO2 and pH?
- Yes, significantly. KH (carbonate hardness) is what buffers pH in an aquarium. Very low KH water (under 2 dKH) has almost no buffering capacity, which means pH can swing dramatically in response to CO2 injection, plant photosynthesis, or respiration. In a planted tank with CO2 injection, the CO2-pH-KH relationship means that low-KH water will show a much larger pH swing during the lights-on period. While low KH is manageable in a stable system, it requires careful CO2 control to avoid dangerous pH crashes. Always check your CO2 levels after significant water changes with RO-blended water.
- What is TDS and does it matter for planted tanks?
- TDS stands for Total Dissolved Solids and measures the total concentration of everything dissolved in your water — minerals, salts, organic matter, CO2, nutrients, and more. It is measured in ppm (parts per million) or mg/L. For planted tanks, TDS itself is less important than GH and KH, which tell you specifically about mineral hardness. However, TDS is a useful monitoring tool: a sudden rise in TDS can indicate a problem (decomposing matter, nutrient buildup), and a TDS reading much higher than you expect from your GH and KH alone suggests other dissolved substances are present. Typical planted tank TDS ranges from 100–400 ppm depending on water hardness and fertiliser use.