Free calculator · no sign-up
RO water calculator for aquariums
How much RO water do you need for the hardness you want? Enter your tap water and target, the calculator shows litres, ratio and the resulting values.
Why one ratio, not two?
RO water contains almost no minerals. Mixed with tap water, GH, KH and conductivity all drop in the same proportion: with 60 % RO water, 40 % of every value remains. So there are no separate mixes for GH and KH, even if some calculators show them.
That is why the calculator starts from GH, the more important value for many animals, such as shrimp when moulting. It shows you the KH and conductivity of the mix as well.
Formula: share of tap water = target GH ÷ tap water GH. At GH 16 and a target of 6 that is 37.5 % tap water and 62.5 % RO water.
What if my tap water is too soft?
Then RO water won’t help. You remineralise with salt (for shrimp, a shrimp salt that raises GH without KH). As a rule of thumb, 0.025 to 0.05 g of salt per litre raises GH by about 1 °dGH. Follow the dosage on the pack.
Starting from pure RO water you create your water entirely yourself. Enter GH 0 for the tap water in that case.
Your water, always matched to your livestock
Aqualizer knows your tap water and what your animals need. If the two don’t match, the app tells you on its own and works out the mix for your next water change.
- Target GH from your animals
- Heads-up when your tap water doesn’t fit
- Step-by-step plan with reminders (Pro)
Coming soon to Google Play – try it now
Join the beta on Android
RO water calculator FAQ
How do I calculate the ratio of RO and tap water?
Share of tap water = target GH ÷ tap water GH, the rest is RO water. At GH 16 and a target of 6 that is 37.5 % tap water and 62.5 % RO water.
Can I set GH and KH separately?
Not by mixing: RO water dilutes both in the same proportion. To raise only one of them you need a suitable salt, for example a GH-only salt for shrimp.
How fast can I change the water parameters in my tank?
Per water change by about 2 °dGH for GH and 1.5 °dKH for KH at most. Bigger jumps stress animals and filter bacteria. The calculator shows how many changes that takes.
Which GH and KH do my fish need?
It depends on the species: soft-water fish such as cardinal tetras or discus like around GH 2 to 8, Malawi cichlids GH 10 to 20. The Aqualizer app works out the target from the animals in your tank.
What conductivity does RO water have?
Good RO water usually has 5 to 20 µS/cm. The calculator simplifies it to 0, so the actual conductivity of the mix is slightly higher.