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Calculate aquarium water volume in gallons and liters for rectangular, bow-front, cylinder, and corner tanks including substrate and displacement.
Calculate CO2 injection rate for planted aquariums from tank volume, target pH, KH carbonate hardness, and bubble rate for stable dissolved CO2.
Check tropical fish compatibility for community tanks by temperament, tank size, water parameters, and schooling needs before adding new species.
One degree German hardness (dGH) equals 17.86 ppm as CaCO3 equivalent. Most aquarium hobbyists use dGH; lab reports often use ppm.
ppm CaCO3 = dGH × 17.86In dilute aqueous solutions, mg/L is numerically equal to ppm. For hardness expressed as CaCO3, mg/L and ppm are interchangeable.
mg/L = ppm (for CaCO3 hardness)Millimolar concentration converts to ppm using the molar mass of CaCO3 (100.09 g/mol). Used in scientific literature and some European test kits.
ppm = mmol/L × 100.09; dGH = mmol/L × 5.61Updated: July 2026
Discus prefer 1–4 dGH (18–71 ppm). Tap water at 8 dGH (143 ppm) needs dilution with RO water. Converter confirms 50/50 mix of 8 dGH tap and 0 dGH RO yields 4 dGH.
Mbuna thrive at 10–20 dGH (179–357 ppm). Tap at 5 dGH needs buffering. Crushed coral or aragonite substrate raises hardness — verify with converter after changes.
Most aquarium plants tolerate 3–12 dGH. At 6 dGH (107 ppm), both CO2 and nutrient dosing work well. Converter helps compare API test kit (dGH) with TDS meter readings.
KH affects pH stability; GH affects osmoregulation and breeding. Test both separately. A tank can have KH 8 and GH 3 — common in soft-water setups with carbonate buffering.
Confirm whether ppm is reported as CaCO3 equivalent (standard) or as elemental Ca²⁺. Elemental calcium ppm is roughly 40% of CaCO3 equivalent ppm.
Aquarium test kits report hardness in different units — dGH, ppm, mg/L, and mmol/L — causing confusion when comparing species requirements. This converter translates between all common hardness scales so you can match water parameters to fish and plant needs.