BRS Pharma-Grade Kalkwasseris a calcium hydroxide (Ca(OH)2) supplement that maintainscalcium,alkalinity, andpHin 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.
The classic three-in-one reef supplement.One dose of saturated limewater raisescalcium,alkalinity, andpHtogether. Pharma-grade purity — no phosphate, silicate, or other reef-tank contaminants. Drip it into your sump as ATO replacement and skip the two-part dosing pumps.
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 hydroxidedissolved 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
3-in-1 Reef SupplementRaises Ca, KH, and pH simultaneously in stoichiometric balance.Pharmaceutical GradeUltra-pure calcium hydroxide — no phosphate or silicate contaminants.Drips Into ATOReplaces evaporation while dosing — no separate pumps needed.Phosphate PrecipitationHigh-pH limewater binds inorganic PO4, starving nuisance algae.Economical2 tsp/gal saturated solution — a 2 lb jar dosed daily lasts months.Reefer-ApprovedThe original go-to for SPS-dominant tanks. Used since the 90s.
Choose Your Size
| Spec | 2 lbs | 4 lbs |
|---|---|---|
| Net Weight | 2 lbs (907 g) | 4 lbs (1.81 kg) |
| Approx. Limewater Yield | ~700 gal saturated | ~1,400 gal saturated |
| Best For | 20 – 90 gal reef tanks | 90+ gal reefs / heavy SPS demand |
| SKU | 210812 | 210813 |
How to Mix & Dose
1. Mix saturated limewater.Add2 teaspoons of kalkwasser per gallon of RO/DI waterin a sealed container (an ATO reservoir works perfectly). Shake or stir; the powder will not fully dissolve — that’s normal. The clear liquidabovethe 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. Doseonlyat night for tanks with low CO2at 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
| Specification | Value |
|---|---|
| Brand | Bulk Reef Supply (BRS) |
| Product | Pharma-Grade Kalkwasser |
| Active Ingredient | Calcium hydroxide — Ca(OH)2 |
| Form | Dry powder |
| Available Sizes | 2 lbs, 4 lbs |
| Use | Saltwater / reef calcium & alkalinity supplement |
| Dosing | 2 tsp per gal RO/DI water; drip via ATO |
| Compatibility | Saltwater only — NOT for freshwater |
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-CO2reef, 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 CO2is 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 lasts4 – 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.
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