Why is KH & Ca Management Important in Reef Tanks?
For corals to build their skeletons (calcium carbonate: CaCO₃), adequate KH (carbonate hardness/alkalinity) and calcium (Ca) must be continuously supplied in the tank water.
- KH target: 8–12 dKH (7–9 dKH for SPS-priority tanks)
- Ca target: 380–450 mg/L (ppm)
- Mg (magnesium) target: 1,250–1,350 mg/L
These parameters are consumed daily and cannot be adequately replenished through water changes alone. Large reef tanks and high-load SPS systems require dedicated supplementation systems.
This article provides a detailed explanation of the two most widely used supplementation methods—kalkwasser and two-part additives—including their mechanisms, setup, and how to choose between them.
---
What is Kalkwasser?
Mechanism and Characteristics
Kalkwasser, German for "limewater," is a saturated solution of calcium hydroxide (Ca(OH)₂) dissolved in pure water (RO/DI water).
When added to the tank, the following reaction occurs:
```
Ca(OH)₂ + CO₂ → CaCO₃ + H₂O
```
- Ca supplementation: Adds approximately 400–450 mg/L of Ca
- KH increase: Consumes CO₂, slightly raising pH and contributing to KH
- Phosphate (PO₄) precipitation: Calcium hydroxide binds with phosphate, improving coral polyp appetite
- pH stabilization: Suppresses nighttime pH drops (CO₂ accumulation)
Limitations of Kalkwasser
- Cannot supplement Ca and KH in equal quantities simultaneously (consumption ratio is approximately 1:2.8)
- Overdosing (pH >8.5) damages corals and fish
- High-load SPS tanks often require more supplementation than kalkwasser alone can provide, necessitating combined use with other methods
---
Kalkwasser Setup and Usage
What You'll Need
- Calcium hydroxide powder (Seachem Reef Kalkwasser, etc.)
- RO/DI water (demineralized pure water)
- Kalkwasser reactor (mixing container) or storage tank
- ATO (Automatic Top-Off device)
Mixing Procedure
- Add approximately 2–5 g of calcium hydroxide per 1 liter of RO/DI water
- Stir well and let stand for 30–60 minutes, using only the supernatant (discard sediment)
- The supernatant Ca concentration is approximately 500–700 mg/L
Note: Calcium hydroxide powder is irritating to skin and mucous membranes. Gloves and safety goggles are recommended.
Automatic Addition via ATO
The safest dosing method is using kalkwasser solution as the water source in your ATO (Automatic Top-Off) system.
- Kalkwasser is added as the ATO automatically replenishes evaporated water
- The slow addition rate prevents rapid pH spikes
- Daily dosing of 1–3% of tank volume is the safe limit
When using a dosing pump (peristaltic pump):
- Set each dose to 50–100 ml or less
- Avoid nighttime dosing; add small amounts during photoperiod so photosynthesis can offset pH increases
---
What is Two-Part Additive?
Mechanism and Characteristics
Two-part, as the name suggests, uses two liquid additives:
- Part A (baking soda-based / NaHCO₃ or Na₂CO₃-based): Supplements KH (alkalinity)
- Part B (calcium chloride-based / CaCl₂-based): Supplements Ca
Representative products:
- BRS (Bulk Reef Supply) 2-Part: Wholesale-oriented, cost-effective
- ESV B-Ionic: Established balanced two-part
- Salifert All-in-One: Multi-formula with trace elements
Advantages
- Adjust KH and Ca independently (easy to correct consumption imbalances)
- Relatively simple setup and calculations
- No calcium reactor needed
- Standard for medium to large tanks (200–600 L)
Disadvantages
- Sodium and chloride accumulate, requiring increased water change frequency in high-load tanks with heavy dosing
- Risk of altered salinity composition with heavy dosing (>10 ml/100 L per week)
---
Two-Part Setup and Calculation
Calculating Required Dosage
Basic calculation formula (daily supplementation guide):
```
KH dosage (ml) = KH consumption (dKH/day) × tank volume (L) × conversion factor
Ca dosage (ml) = Ca consumption (mg/L/day) × tank volume (L) × conversion factor
```
Example (300 L reef tank, high-load SPS):
- KH consumption: 0.1 dKH/day → 3 ml/100 L daily → 9 ml/day
- Ca consumption: 5 mg/L/day → 2 ml/100 L daily → 6 ml/day
※ Conversion factors vary by product, so prioritize the product label instructions.
Automatic Addition with Dosing Pump
Optimal practice is to use a commercial dosing pump (Kamoer, Neptune DOS, etc.) and dose 2–4 times daily in small amounts.
- Per-dose amount: 0.1% of tank volume or less (≤300 ml for 300 L tank)
- KH can be dosed at night and Ca during the day (though water quality impact is minimal)
---
Kalkwasser vs Two-Part: Which Should You Choose?
| Comparison | Kalkwasser | Two-Part |
|---|
| Startup cost | Low (powder + ATO) | Medium (2 bottles + pump) |
| Monthly cost | Low (inexpensive powder) | Medium–High (reduced with bulk purchase) |
| pH stabilization | Yes (nighttime pH support) | No |
| Independent KH/Ca adjustment | Difficult | Easy |
| Best for | ~300 L, medium load | 200–500 L, medium–high load |
| Setup difficulty | Medium (watch for overdosing) | Low–Medium |
Combined Use (Advanced)
For high-load SPS tanks, combined use of kalkwasser (via ATO) + two-part supplementation is effective:
- Kalkwasser provides base supplementation during evaporation makeup and phosphate control
- Two-part precisely supplements any KH/Ca deficiency
- Consider switching to a calcium reactor if consumption is extremely high
---
Practice: Initial Setup and Weekly Management Flow
Initial Setup Procedure
- Baseline measurement: Measure KH, Ca, and Mg (ideally at the same time over 2 days)
- Calculate consumption rate: Calculate the change after 24 hours
- Set dosage: Consumption rate × tank volume = daily dosage
- Start low: Begin at 50–70% of calculated amount and readjust after remeasuring in one week
Weekly Checklist
| Parameter | Target | Frequency |
|---|
| KH | 8–11 dKH | 2–3 times/week (SPS: daily) |
| Ca | 380–450 ppm | Twice/week |
| Mg | 1,250–1,350 ppm | Once/week |
| pH (daytime peak) | 8.1–8.3 | Automatic monitoring recommended |
---
Summary: Continuous Measurement and Adjustment Matter More Than Method Choice
Rather than which method is superior, the most important thing is to choose the method that suits your tank size, consumption rate, and management style, and develop a habit of continuous measurement and adjustment.
Either method can achieve stable coral keeping with proper operation. If unsure, start with two-part to master the basics, then transition to kalkwasser or a calcium reactor as your tank grows or your management becomes more advanced.
