Kalkwasser has been a staple in reef keeping for decades, and for good reason. This simple mixture of calcium hydroxide and water can help you maintain stable calcium, alkalinity, and pH with minimal equipment and cost. Used correctly, kalkwasser becomes a powerful tool for growing healthy corals and coralline algae in your reef tank.

What Is Kalkwasser and Why Use It?

Kalkwasser (German for “lime water”) is a saturated solution of calcium hydroxide in fresh water. When dosed into your reef aquarium, it delivers two key elements your corals consume daily:

• Calcium – Essential for coral skeleton growth and coralline algae.
• Alkalinity – Helps stabilize pH and supports overall reef health.

Unlike many other additives, kalkwasser adds calcium and alkalinity in a balanced ratio, so it doesn’t skew your parameters in one direction. It also raises pH slightly, which is especially helpful in tanks that struggle with low pH due to indoor CO₂. If you’re new to dosing, you might want to review your reef tank water parameters first so you know your baseline.

How to Mix and Dose Kalkwasser Safely

Getting kalkwasser right is less about complexity and more about consistency. Here’s a simple approach that works for most hobbyists:

Mixing Kalkwasser

• Start with RO/DI water in a clean container with a lid.
• Add 1–2 teaspoons of calcium hydroxide per gallon of water.
• Stir briefly, then let it sit for 30–60 minutes so the undissolved powder settles.
• Use only the clear solution from the top; leave the white sludge at the bottom.

Label the container and keep it sealed to minimize CO₂ exposure, which will weaken the solution over time.

Dosing Methods

The safest way to use kalkwasser is to add it slowly and consistently. Popular methods include:

• Auto top-off (ATO) integration – Replace your ATO reservoir with kalkwasser. As water evaporates, kalk is added gradually.
• Dosing pump – Drip kalkwasser into a high-flow area of the sump over several hours, usually at night to help stabilize pH.
• Manual drip – A simple gravity drip system for smaller tanks or testing the method before automating.

Pro tip: Always add kalkwasser slowly. Large, fast additions can cause pH spikes and stress your fish and corals.

Best Practices, Troubleshooting, and When to Upgrade

Kalkwasser works best on small to medium systems or lightly to moderately stocked SPS and mixed reefs. As coral growth takes off, your demand may outgrow what kalk alone can supply.

Best Practices

• Test alkalinity and calcium weekly when starting out, then adjust your concentration or dosing rate.
• Monitor pH, especially if you’re dosing at night, to avoid overshooting.
• Keep your kalk container shaded and sealed to prolong potency.
• Clean ATO lines and pumps regularly to prevent buildup.

Common Issues

• Cloudy tank water: Usually from dosing too fast or adding undissolved powder. Slow down the drip and use only clear solution.
• Rising alkalinity: Reduce the saturation (use less powder) or shorten dosing time.
• Insufficient levels: If your tank’s demand is high, consider combining kalkwasser with a two-part dosing system or a calcium reactor.

For many reef keepers, kalkwasser is the perfect first step into automated dosing. It’s inexpensive, effective, and pairs well with a simple reef tank maintenance schedule. As your coral colonies grow and your tank matures, you can always layer in more advanced methods while still using kalk to support pH and baseline demand.

Used thoughtfully, kalkwasser can become one of the most reliable tools in your reef-keeping toolbox, helping you maintain rock-solid stability and fueling strong, steady coral growth.