Calculate the mixing ratio of RO and tap water for your target GH and KH – right here, no sign-up needed.
Formula: RO share (%) = 1 – target GH ÷ tap water GH
Why do GH and KH differ? GH and KH aren't always proportional in tap water. If the GH target requires 50% RO but the KH target requires 60% – you can only hit one exactly through mixing. The other is corrected with a mineral additive (GH booster or KH booster).
| Fish species | Target GH | Target KH |
|---|---|---|
| Discus / tetras | 2–6 dGH | 0–3 dKH |
| South American cichlids | 4–10 dGH | 2–5 dKH |
| General tropical fish | 8–15 dGH | 4–8 dKH |
| Malawi cichlids | 12–20 dGH | 6–12 dKH |
| Shrimp (Neocaridina) | 6–15 dGH | 4–8 dKH |
| Shrimp (Caridina) | 2–6 dGH | 0–2 dKH |
Maintaining your RO unit: replace filter membranes every 1–2 years, pre-filters every 3–6 months. A depleted membrane won't produce true RO water.
Right now the calculator always uses whatever you type in. With an account, your tap water's GH, KH and conductivity are saved and pre-filled automatically.
Enter your tap water's GH, KH and conductivity once – the calculator fills it in automatically from then on.
How much RO water you need per change is calculated automatically. Reminders keep you on schedule.
Measure and log after every water change – you'll see instantly whether the mix ratio is right or needs adjusting.
After the water change: a step-by-step guide through the whole process – with the exact mix ratio for your tank.
Log every reading and see in the history how stable your water is – and when a correction is needed.
Which mineral supplement (GH booster, KH booster, salts) fits? The CO2 calculator and RO calculator work together.
In the stocking planner, GH/KH requirements are stored for every fish species – the target value is suggested automatically.
Formula: RO share = 1 – (target GH ÷ tap water GH). Example: tap water GH 16, target GH 8 → 50% RO + 50% tap water. This result is for GH – KH needs to be checked separately.
GH and KH rarely rise proportionally in tap water. Calcium (the GH component) and carbonates (the KH component) come from different sources. If GH requires 50% RO but KH requires 60%, you can only hit one exactly through mixing – the other is adjusted with a mineral supplement.
Only if your tap water is significantly too hard for the livestock you are planning. For most tropical fish, regular tap water is fine. For discus, Caridina shrimp, or sensitive soft-water fish, RO water makes sense.
GH specifically measures calcium and magnesium ions in dGH. Conductivity (µS/cm) measures all dissolved ions combined. As a rough rule of thumb: µS/cm ÷ 30 ≈ dGH (only an approximation, since tap water composition varies).
Tap water changes seasonally and after maintenance work on the water network. Compare GH and KH from your latest measurement with your water utility's annual report. In Aqualizer you can update your tap water profile whenever needed.
Save your tap water profile, keep your mixing ratio on hand, track water changes, and set reminders – for free.
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