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Redmoon Converters
🐾 Pets & Animals

Aquarium Water Parameter Converter

Convert any aquarium water parameter — ppm to dGH / dKH, °C / °F and meq/L — with safe-range presets for common freshwater and marine species.

Built and maintained by Paul Clark, Redmoon Software · Checked against what this tool computes on

Hardness conversions
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UnitValueTypical use
Quick reference

1 dGH = 17.86 ppm CaCO₃ = 0.357 meq/L. Same conversion applies to dKH (carbonate hardness) because both are reported as calcium-carbonate equivalent. ppm and mg/L are interchangeable for dilute aquarium water.

Temperature: °C = (°F − 32) × 5/9. Most freshwater tropicals sit at 24–27 °C (75–80 °F). Discus prefer 28–30 °C; goldfish 18–22 °C. Always match new water to existing tank temperature within 1 °C to avoid stressing fish.

How it works

Every hardness and alkalinity conversion routes through ppm as calcium carbonate as a pivot value. Hardness and alkalinity share the same constants because both are reported as calcium-carbonate equivalent: one dGH or dKH equals 17.86 ppm, one meq/L equals 50.04 ppm, and one grain per US gallon equals 17.118 ppm. Temperature is handled on its own track with the exact definitions, °C = (°F − 32) × 5/9 and K = °C + 273.15, rounded to two decimals.

Choose Parameter first, because it rebuilds the Source unit list. Hardness (GH / CaCO₃) offers ppm, °dGH, meq/L and grains per US gallon; Alkalinity (KH) drops the grains option and keeps ppm, °dKH and meq/L; Temperature offers °C, °F and K. The default is 100 ppm hardness, which converts to 5.60 °dGH. The four preset buttons print a target-range reminder for that setup rather than changing any of your inputs.

Those presets are display-only text and any recalculation clears them, so read the range before you convert or press the button again afterwards. There is no pH conversion at all, even though the preset lines quote pH ranges, and no salinity or TDS handling despite the reef preset mentioning specific gravity. The conversions also assume your test kit reports on a calcium-carbonate-equivalent basis, which is standard but worth confirming on the box.

Frequently asked questions

How do I convert ppm to dGH?

Divide by 17.86. A water report showing 100 ppm as calcium carbonate is 5.60 dGH, and 180 ppm is 10.08 dGH. The results table lists ppm, dGH, meq/L and grains per US gallon together so you can read across from whichever kit you own.

Is dKH converted the same way as dGH?

Yes. Both are expressed as calcium-carbonate equivalent, so the same 17.86 ppm per degree applies. Selecting Alkalinity relabels the output and drops grains per US gallon from the unit list, since water-softener units are not used for carbonate hardness.

What hardness should a tropical community tank have?

Pressing Tropical community prints 5–12 dGH, 4–8 dKH, pH 6.5–7.5 and 24–27 °C. Three more buttons cover African cichlids, discus and reef. These are reference ranges shown beneath the result and they do not alter any input value.

What is meq/L and when would I use it?

Milliequivalents per litre is the laboratory and reef-chemistry unit, where one meq/L equals 50.04 ppm as calcium carbonate. Reef keepers usually see alkalinity quoted in dKH or meq/L, and dosing calculators frequently expect meq/L rather than a ppm figure.

Why does the preset range vanish when I type a value?

The preset text is written into a separate line that every recalculation clears, so it disappears the moment you change the parameter, source unit or value. Note the range down before converting, or simply press the preset button again afterwards.

Hardness, alkalinity and the units that get confused

GH measures general hardness — mostly calcium and magnesium — while KH measures carbonate hardness, which is the water's buffering capacity against pH swings. They are different measurements and are routinely conflated, partly because both are reported in degrees and partly because tap water often has both.

KH is the one that keeps a tank stable. Low KH means pH can crash quickly, which is far more dangerous to fish than a pH that is stable at a value slightly outside their ideal range.

Changing parameters slowly

Fish tolerate a wide range of stable conditions and tolerate rapid change badly. Adjusting hardness or pH quickly is more harmful than leaving it where it is, so the usual advice is to select species suited to your tap water rather than to engineer the water to suit the species.

Where adjustment is necessary, do it in the change water rather than the tank, and spread it over multiple water changes. Chemical pH adjusters that do not also address KH produce a value that drifts back and a tank that swings, which is the worst of both.

Temperature conversions are exact; everything else here is a guide to be confirmed with a test kit on your own water.

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