Deltec KM500 Kalkwasser Stirrer
Deltec (D-D The Aquarium Solution)
A sealed reactor that keeps a bed of calcium hydroxide powder gently stirred at the bottom, so auto top-off water passes through and leaves as clear, saturated limewater at about pH 12.4.
Why it mattered
Kalkwasser is among the oldest calcium methods in the hobby, credited to Peter Wilkens, and it doses calcium and alkalinity in one solution while raising pH. Its weakness is freshness: saturated limewater holds roughly 900 mg/L calcium and degrades quickly on contact with air.
The stirrer design answers that weakness. Top-off water enters the sealed chamber, the stirrer keeps fresh powder available at the base, and clear saturated solution exits from the top — every top-off event delivers full-strength kalk without a mixing bucket.
The Deltec KM500 (and its larger T variant, rated for systems up to about 1,575 L) became the reference stirrer. Paired with an ATO, it turned the hobby's oldest supplement into an automated system, and kalk has outlived several generations of dosing fashion since.
Key specs
- Sealed chamber limits CO2 contact and potency loss
- Saturated limewater: ~pH 12.4, ~900 mg/L calcium
- Stirrer keeps the powder bed fresh without clouding output
- KM500T rated for systems up to ~1,575 L (350 gal)
- Driven by the auto top-off, so dosing follows evaporation
Keeper notes
- Some keepers measured stirrer output as under-saturated and went back to batch mixing in a still reservoir.
- Others have run a KM500 for years and report steady alkalinity with nothing more than powder refills.
- Because dosing follows evaporation, seasonal swings in evaporation rate are the thing to watch.