Why do GH and KH give different mixing ratios?

GH and KH rarely rise in the same proportion in tap water. If the mixing ratio for GH and KH differs, only one target can be hit through mixing – the other value is corrected with a mineral supplement or buffer salt.

How often do I need to prepare RO water for the aquarium?

With a weekly 30% water change on a 200 L (53 gal) tank you need 60 L (about 16 gal) of treated water per week. Aqualizer lets you track this and set reminders.

What is the difference between GH and KH in an aquarium?

GH (general hardness) measures calcium and magnesium ions. KH (carbonate hardness) measures carbonates and bicarbonates and buffers pH. Both parameters affect fish and plant health differently.

What GH and KH do tropical fish need?

Discus and tetras: GH 2–8, KH 1–4. Lake Malawi cichlids: GH 12–20, KH 6–12. General tropical fish: GH 8–15, KH 4–8. Always check the requirements of your specific species.

Aquarium RO Water Calculator – Mix GH & KH Correctly

Q: How much RO water do I mix for my aquarium?

Mixing ratio = target GH ÷ tap water GH. Example: tap water GH 16, target GH 8 → 50% RO + 50% tap water.

Inputs

Tap water

GH (dGH)

KH (dKH)

µS/cm (opt.)

Target values

Target GH (dGH)

Target KH (dKH)

Volume

Target volume (L) water change or tank volume

Result

Enter tap water and target values.

Help – understanding RO water mixing

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.

With a free account

Save your tap water profile once – use it forever.

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.

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Save your tap water profile

Enter your tap water's GH, KH and conductivity once – the calculator fills it in automatically from then on.

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Water change reminders

How much RO water you need per change is calculated automatically. Reminders keep you on schedule.

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GH & KH history

Measure and log after every water change – you'll see instantly whether the mix ratio is right or needs adjusting.

More features

Track your entire water treatment routine

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RO water plan

After the water change: a step-by-step guide through the whole process – with the exact mix ratio for your tank.

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GH/KH history

Log every reading and see in the history how stable your water is – and when a correction is needed.

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Remineralization

Which mineral supplement (GH booster, KH booster, salts) fits? The CO2 calculator and RO calculator work together.

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Species-specific targets

In the stocking planner, GH/KH requirements are stored for every fish species – the target value is suggested automatically.

Why the calculator helps

Guessing vs. calculating

Without a calculator

✗Mixing ratio by feel

✗GH too high – shrimp stressed

✗KH too low – unstable pH

✗Guessing again every time

With Aqualizer

✓Exact mixing ratio in seconds

✓GH and KH cross-checked instantly

✓Tap water profile saved

✓History and reminders included

Frequently asked questions

How do I calculate the mixing ratio for my aquarium?

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.

Why do GH and KH mixing ratios differ from each other?

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.

Do I really need a reverse osmosis unit?

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.

What is the difference between GH and conductivity?

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).

How often should I check the mixing ratio?

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.

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Your RO water plan, ready for your tank

Save your tap water profile, keep your mixing ratio on hand, track water changes, and set reminders – for free.

Free · No credit card · No download

RO Water Calculatorfor Aquariums