How much reverse-osmosis, rainwater or distilled water to blend with hard tap water to reach a softer target hardness. Handy for shrimp, many tetras, apistos and other soft-water species.
You cannot chemically strip hardness out of tap water; you can only dilute it with a near-pure source such as reverse-osmosis, rain or distilled water, or add hardness back. So softening is always a blend. Measure your tap first, because it drifts through the year, then decide a target that suits your fish: many tetras, apistos and Caridina shrimp want soft water, while livebearers and Rift Lake cichlids want it hard. To go the other way and build soft water up, use the remineralisation calculator, and read keeping fish in hard water.
The fraction of tap is (target minus RO) over (tap minus RO). This calculator gives a ratio and litres of each, treating RO, rain and distilled water as zero hardness.
Yes. Clean rainwater and distilled water are close to zero hardness and stand in for RO. Let rainwater settle and check it if you are unsure of contamination.
Dilute it with a near-zero-hardness source. Salts only add hardness, so softening is always a dilution. Measure your tap first, since it varies.
This assumes hardness dilutes linearly, which is a good approximation for GH: the fraction of tap water needed is (target − RO) ÷ (tap − RO). RO, rain and distilled water are treated as effectively zero hardness. Real tap water varies through the year, so measure yours rather than trusting the utility's average. The 22.5 °dH default is the local very hard SWDE tap figure around Tubize. 1 °dH = 17.86 ppm as CaCO₃. To go the other way and build soft or RO water up to a target GH and KH, use the remineralisation calculator.