The RO-to-tap ratio comes from one chosen target
To mix RO and tap water for a chosen aquarium hardness, first calculate the tap fraction: f = (target − RO reading) / (tap reading − RO reading). Multiply the preparation volume by f for tap water and by 1 − f for RO water. Use readings of the same property in the same units. This volume-weighted model estimates a separate batch; it does not select suitable water conditions or tell you how quickly to change an occupied tank.
A statement such as one part tap to two parts RO means three total parts, with tap supplying one third of the batch. It does not mean that tap supplies half. Label the order whenever writing a ratio, because RO-to-tap reverses tap-to-RO. The useful sequence is measurement, chosen target, calculated fractions, then measured quantities. A familiar ratio from another aquarium is not a substitute for testing the sources available to you.
Measure both sources before choosing the calculation
Record tap GH, tap KH, RO GH, and RO KH separately. GH and KH are not alternate labels for one reading. A source that tests low in one parameter does not automatically supply a known value for the other. RO water is often low in dissolved minerals, but the calculator does not assume every membrane and stored batch produces zero hardness. Enter actual measurements where the test can resolve them, and retain uncertainty when it cannot.
The tool expects GH in dGH and KH in dKH. If a compatible report uses mg/L as calcium-carbonate equivalents, normalize that reading with the hardness converter before entering it. Do not use a TDS meter number as a hardness result or feed a ppm reading directly into a degrees field. Keep sampling and test procedures consistent, particularly when checking whether a changed mixing result comes from different source water rather than a changed ratio.
Worked GH example: prepare a 60-liter batch
Suppose measured tap water is 15 dGH and 9 dKH, while measured RO water is 1 dGH and 1 dKH. For an illustrative GH target of 8 dGH, the tap fraction is (8 − 1) / (15 − 1) = 7 / 14 = 0.5. In a 60 L preparation batch, tap contributes 60 × 0.5 = 30 L and RO contributes the other 30 L. These chosen numbers demonstrate arithmetic, not a recipe for a species.
Check the calculation by rebuilding the blend: (30 × 15 + 30 × 1) / 60 = 8 dGH. The quantities add to 60 L, and the result lies between the two source readings. The same ratio gives an estimated KH of (30 × 9 + 30 × 1) / 60 = 5 dKH. That second result is an outcome of the GH-controlled blend, not an additional condition that was independently solved.
Why one blend cannot always meet both GH and KH targets
Now keep those sources and batch volume but choose KH as the controlling parameter, with an illustrative target of 3 dKH. The tap fraction becomes (3 − 1) / (9 − 1) = 0.25. The batch needs 15 L tap and 45 L RO. Its calculated GH is 0.25 × 15 + 0.75 × 1 = 4.5 dGH. Changing the controlling target changes both quantities and both resulting readings.
The desired pair GH 8 and KH 3 therefore cannot be made from these two sources by ratio adjustment alone. GH 8 requires a tap fraction of one half; KH 3 requires one quarter. No single fraction satisfies both. The calculator deliberately exposes this conflict by showing the secondary reading. It does not quietly introduce a buffer or remineralizer, and this guide does not provide dosing instructions to force a desired pair.
Check endpoints, equal sources, and impossible targets
A blend of two source readings stays between them under this model. With GH 15 and GH 1, a target of GH 18 or GH 0 cannot be reached by mixing alone. A target of GH 15 uses entirely tap water; GH 1 uses entirely the measured RO source. The equation can describe those endpoints, but mathematical acceptance does not establish whether either source is appropriate for the animals or the intended maintenance operation.
Equal source readings create a different problem. If both sources measure 6 dGH and the target is 6, every blend meets that one constraint and there is no unique ratio. If the target is 4, no blend works. The interface reports these situations instead of dividing by zero. Recheck parameter labels and unit conversions before concluding that equipment has failed or that a chemical adjustment is needed; an entry mismatch can create the same apparent contradiction.
Scale the batch without changing the physical ratio
For unchanged source readings and target, scaling only changes the quantities. A 0.5 tap fraction needs 10 L tap in a 20 L batch, 30 L in a 60 L batch, and 50 L in a 100 L batch. You do not need a new percentage merely because the container changes. Conversely, adding an unrecorded extra amount of either source changes the final ratio, so the intended preparation volume and actual quantities should agree.
Liters, US liquid gallons, and imperial gallons can all describe the same preparation. One US liquid gallon is 3.785411784 L; one imperial gallon is 4.54609 L. Changing the tool's volume unit converts the batch amount while preserving the blend fraction. Check the bucket's convention rather than using gallon without a qualifier. Unit conversion cannot remove uncertainty in a poorly marked container, and rounded display quantities should not be mistaken for perfect measurements.
Do not extend a hardness average into a pH prediction
The model assumes the selected hardness or alkalinity reading behaves approximately as an additive quantity over the batch volumes. It does not simulate chemical reactions, equilibration, or changes caused by substances introduced afterward. Test the prepared batch rather than treating its displayed GH and KH as guaranteed analytical results. If source readings were approximate, the mixture prediction inherits that uncertainty even when the fraction has a tidy decimal or whole-number ratio.
Do not average pH values with this equation. pH is logarithmic, while buffering and chemical composition influence how mixed water behaves. Matching GH alone also says nothing complete about disinfectants, temperature, salinity, or trace substances. Those are separate preparation and husbandry questions. This guide is educational, not animal-care advice, and does not choose a species target, authorize untreated tap water, or recommend the size and timing of a change to an established aquarium.
Use the calculator as a checkable preparation worksheet
Open the Aquarium RO Water Mixing Calculator, choose a volume unit, and enter the batch size. Select the parameter that will determine the ratio, enter the four measured source readings, and supply that parameter's chosen target. Read tap and RO quantities together, then inspect the estimated secondary parameter. The four result cards form one calculation, while tap share below them is supporting information. Missing or unreachable inputs remove the calculated results rather than preserving an old recipe.
Record the source readings, target, batch quantities, date, and measured finished-batch result outside the tool if you need a reproducible log. The browser calculation does not save a water profile. A separate water-change calculation can determine the quantity associated with a percentage you have already chosen, but it does not validate this blend's chemistry or its suitability for livestock. Keep batch preparation, verification, and decisions about use distinct, rather than interpreting a correct ratio as approval of every later step.