RO water comes out of your reverse osmosis system completely pure, but also completely "dead" (0 GH, 0 KH). To make it safe for fish, shrimp and plants, you need to add minerals back in with a remineralizer, at the right GH:KH ratio for your type of tank (most commonly 4-8 dGH and 4-8 dKH for a general planted community tank, lower for Caridina shrimp and soft-water species). Below is the exact step-by-step process, target values by tank type, and which product to choose.
Contents
- Why RO water has no minerals, and why that's a problem
- GH and KH in short: what they actually measure
- Target GH and KH values by tank type
- Step by step: how to remineralize RO water
- The most common remineralization mistakes
- Frequently asked questions
Why RO water has no minerals, and why that's a problem
Reverse osmosis (RO) strips out virtually everything dissolved in water: chlorine, heavy metals, nitrates, but also the calcium, magnesium and carbonates that GH and KH depend on. The result is water with TDS near zero, which sounds "clean" but in practice can't be poured straight into a tank.
Without calcium and magnesium, fish struggle to regulate their osmotic balance, shrimp can't molt properly, and plants are left without key macronutrients. Without carbonate hardness (KH), water has no buffer, and pH can crash suddenly at the smallest disturbance, especially in CO2-injected tanks.
That's why every RO system, including a higher-output professional unit like the Arka myAqua1900, produces a raw material, not water that's ready to pour in. Remineralization is the mandatory second step.
For a fish room, multiple tanks, or a large high-tech aquascape. Produces pure water that's ready for remineralization using the steps below.
GH and KH in short: what they actually measure
GH (general hardness) measures the dissolved calcium and magnesium in your water. Fish need these minerals for normal metabolism, shrimp need them for healthy molting, and plants use them as key nutrients. Low GH means slow growth, faded colors and molting problems.
KH (carbonate hardness) measures your water's buffering capacity, in other words how resistant your pH is to sudden swings. Low KH isn't automatically bad, but below a certain point you risk a sudden pH crash, especially if you're injecting CO2. One thing worth knowing: adding CO2 on its own doesn't change KH, it only lowers pH temporarily.
Target GH and KH values by tank type
There's no single universal number. Target values depend on what you're keeping:
| Tank type | Target GH | Target KH |
|---|---|---|
| General planted community tank | 4-8 dGH | 4-8 dKH |
| High-tech tank with CO2 injection | 3-6 dGH | 2-7 dKH |
| Caridina shrimp (Crystal Red, Bee) | 4-6 dGH | 0-2 dKH |
| Neocaridina shrimp (Cherry, Blue Dream) | 5-8 dGH | 3-6 dKH |
| Soft-water fish (discus, tetras, rasboras) | 1-4 dGH | 1-3 dKH |
Aim for the middle of the range rather than the extremes. Stability over time matters more than hitting an exact number on any given day.
Step by step: how to remineralize RO water
Step 1: Test your RO output water
Before adding anything, confirm your RO system is actually producing water with GH and KH near zero (a TDS meter is enough for a quick check). If it isn't, the filter membranes likely need replacing.
Step 2: Pick the right remineralizer for your situation
Not every remineralizer does the same job. Some raise only GH, some only KH, and others do both at once:
A pure calcium-magnesium booster for when your KH is already where you want it and you only need to raise general hardness.
A concentrated remineralizer that raises GH and KH at the same time. Safe for shrimp and snails, and a solid default choice for most community tanks.
A liquid adjuster that raises carbonate hardness and stabilizes pH without touching GH. Handy for high-tech CO2 tanks where you want low GH but stable KH.
Purpose-built for RO/RODI water in planted tanks, 2HR Aquarist's flagship remineralizer. A great fit if you're already using the rest of the brand's lineup.
Step 3: Calculate the right dose for your water volume
Every remineralizer comes with its own precise dosing chart (grams or milliliters per 10 or 100 liters), because calcium, magnesium and carbonate concentrations differ between brands. Dose by the package instructions, not by feel, and always round down on your first dose so you can check the effect before adjusting further.
Step 4: Mix thoroughly and let it dissolve completely
Mix the solution in a separate container of RO water until it's fully dissolved before pouring it into the tank or your water-change reservoir. Undissolved particles can cloud the water temporarily or leave residue on your decor.
Step 5: Re-check GH and KH before pouring in
Wait 10 to 15 minutes, then test again. If you're close to the target from the table above, the water is ready. If not, add a small additional amount rather than overdosing all at once.
The most common remineralization mistakes
Mistake 1: Assuming GH and KH are the same thing. They're not. You can have high GH and low KH at the same time (common in shrimp tanks), so test both parameters separately.
Mistake 2: Dosing "by eye" without measuring your water volume. Even a small error in volume causes a big difference in your final GH/KH, especially in smaller nano tanks.
Mistake 3: Relying on CO2 injection alone to "fix" low KH. CO2 lowers pH temporarily, but it doesn't change your water's buffering capacity. Low KH stays low KH.
Mistake 4: Changing too large a volume of water at once with freshly remineralized water. A sharp swing in GH/KH during a water change stresses fish, and shrimp especially. Stick to under 30% per change unless you're adjusting parameters gradually.
Ready to remineralize your RO water the right way
Browse the full range of remineralizers and RO systems at Aquarium Boutique and find the right combination for your tank.
Shop remineralizers →Frequently asked questions
What's the difference between GH and KH?
GH measures dissolved calcium and magnesium (general hardness), while KH measures the carbonates that buffer pH against sudden swings. It's entirely possible to have one high and the other low at the same time.
Can I use tap water instead of RO water with a remineralizer?
You can, if your local tap water is already close to your target values and free of chloramines or heavy metals above safe levels. In most cities, though, tap water is too hard or too inconsistent in composition, which is why RO plus controlled remineralization gives a far more predictable result.
How do I know what GH/KH values my shrimp need?
Check the species: Caridina types (Crystal Red, Bee) want low KH (0-2 dKH) and moderate GH, while Neocaridina types (Cherry, Blue Dream) are more tolerant and prefer higher GH and KH, as shown in the table above.
Will my CO2 system change my KH?
Not directly. CO2 lowers pH temporarily by forming carbonic acid, but it doesn't change your water's buffering capacity (KH). If KH is too low, the risk of a pH crash remains regardless of CO2 injection.
How often should I test GH and KH?
Once a month is enough for a stable tank. Test right away after replacing an RO membrane, when starting a new batch of remineralizer, or if you notice molting problems in shrimp or slowed plant growth.