RO and tap water mixing calculator
Cut hard tap water with RO water to reach a softer target. Enter what your meter or test kit reads for each source, the reading you want, and how much water you are making. You get the volume of tap and of RO.
How the mix works
The reading of a blend is the volume-weighted average of its two sources. Ultimate Finance Calculator writes it as (TDS_target × V_total) = (TDS_tap × V_tap) + (TDS_ro × V_ro). Solving for the tap volume gives V_tap = V_total × (target − RO) / (tap − RO), and the RO volume is whatever is left.
A UKAPS forum answer says the same in words: as you mix tap and RO, "conductivity, hardness and alkalinity will all decline in direct proportion to the amount of RO water added", (UKAPS thread). By my arithmetic, a 50:50 mix gives half of the tap water's values. That is why the same formula works for TDS, GH and KH, as long as you enter the same kind of reading for both sources and the target.
Worked example
You want 10 gal at 100 ppm. Your tap reads 300 ppm and your RO reads 0 ppm. The tap share is (100 − 0) / (300 − 0), one third. That is 3.33 gal of tap and 6.67 gal of RO.
Load this example into the calculator.
TDS does not give you a GH
A TDS meter counts everything dissolved, which includes the minerals that make up hardness and much else. Shrimp Science says to focus on getting the right general hardness and carbonate hardness rather than a specific TDS, and that TDS gives a rough idea but is not too helpful by itself. It gives 1 dGH as equal to 17.8 ppm, and notes that with its own remineralizer 1 dGH is closer to 20 ppm.
If your goal is a GH, mix by a GH reading from a test kit and check the result. If you mix by TDS, treat the dGH you end up with as an estimate. The tap and RO readings also drift, so measure each source before a big batch.
Going above your tap water
A blend of tap and RO can only land between the two readings. Shrimp Science explains how to add minerals to RO water for a higher target in how to remineralize shrimp tank water. Follow the dose on your remineralizer's own label.
Tap water going into the tank still needs conditioner: see the dechlorinator calculator. For how much to swap, use the water change calculator or the shrimp tank page.