BRS Kalkwasser | Calcium Hydroxide Reef Supplement (Ca + KH + pH)

BRS Pharma-Grade Kalkwasser is a calcium hydroxide (Ca(OH)₂) supplement that maintains calcium, alkalinity, and pH in saltwater reef aquariums in a single dose. Mix with RO/DI water to make "limewater," then drip it into your sump to replace evaporated water — the limewater simultaneously tops off your tank, raises pH, and replenishes the calcium and carbonate stony corals consume to build skeleton. Browse all reef supplements & additives or shop saltwater supplies.

What Kalkwasser Does

In a stocked reef tank, hard corals (SPS, LPS, clams) constantly pull calcium and carbonate out of the water to build skeleton. If you don't replace what they consume, calcium and alkalinity drop — the corals stop growing and eventually start receding. Kalkwasser is the simplest fix: calcium hydroxide dissolved in your top-off water. Each gallon of limewater delivers roughly equal molar amounts of Ca²⁺ and OH^-, which means it raises calcium and alkalinity at the same ratio corals consume them. Bonus: the high-pH limewater also precipitates inorganic phosphate, keeping nuisance-algae fuel low.

Features

Choose Your Size

How to Mix & Dose

1. Mix saturated limewater. Add 2 teaspoons of kalkwasser per gallon of RO/DI water in a sealed container (an ATO reservoir works perfectly). Shake or stir; the powder will not fully dissolve — that's normal. The clear liquid above the settled powder is your saturated limewater (~2 g/L CaO equivalent).

2. Let it settle. Wait 1 – 2 hours for the undissolved powder to fall to the bottom. Only the clear liquid above gets dosed — never agitate the bottom sediment into your dosing line.

3. Drip into the sump. Use the limewater as your top-off water. A standard ATO with a slow drip into a high-flow area of the sump is ideal. Dose only at night for tanks with low CO₂ at lights-on, or 24/7 for stable systems.

4. Refill when clear. When the liquid turns completely clear (no more white powder at the bottom), the calcium hydroxide has been consumed. Add more kalkwasser and repeat.

Specifications

Recommended Pairings

• An RO/DI system or RO/DI water — kalkwasser MUST be mixed with purified water, never tap
• An alkalinity & calcium test kit — dose to a target, not a guess
• Browse all reef supplements & additives
• Browse saltwater supplies

FAQ

Kalkwasser vs. two-part dosing — which should I use?
Kalkwasser is great for low-to-moderate demand reefs (mixed reef, soft corals, beginner SPS), and it has the unique benefit of precipitating phosphate. Two-part (calcium + alkalinity solutions dosed separately) scales to any consumption rate without the pH ceiling kalkwasser hits. Many advanced reefers use BOTH: kalkwasser as the main ATO method, two-part to top up the difference. For tanks demanding more than a few dKH/day, two-part is needed.

Will it raise my pH too high?
Limewater is high-pH (~12 in concentrated form, but heavily diluted as it enters the tank). In a well-aerated, low-CO₂ reef, kalkwasser typically raises tank pH from ~7.9 to a healthy 8.2 – 8.4. If pH spikes above 8.4, slow the drip rate or dose only at night when respiratory CO₂ is highest.

Can I dose it directly into the tank?
No. Limewater must drip slowly into a high-flow area (typically the sump). Direct dosing into the display will cause localized pH spikes that can burn corals and stress fish.

How long does a 2 lb jar last?
For a typical 40 – 75 gallon reef tank evaporating ~1 gal/day, a 2 lb jar of kalkwasser lasts 4 – 6 months. The 4 lb jar lasts roughly twice as long. The exact duration depends on calcium demand from your coral load.

Is it safe for freshwater tanks?
No. Kalkwasser is for saltwater reef use only. The pH and calcium levels it produces are not appropriate for freshwater systems.