Kalkwasser dosing in reef tanks: how to mix, dose, and raise pH safely

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Kalkwasser (German for "limewater," usually shortened to kalk) is calcium hydroxide dissolved in fresh RO/DI water. It feeds a reef tank calcium and alkalinity in the same ratio corals use to build skeleton, and it pushes pH up at the same time. The catches are that it is strongly caustic, that dosing it too fast spikes pH, and that the amount you can add each day is capped by how much water your tank evaporates. This guide works through the numbers so you can tell whether kalk fits your tank and how to run it safely.

What kalkwasser is

Lime in the chemical sense means quicklime (calcium oxide) or hydrated lime (calcium hydroxide, also called slaked lime). Quicklime turns into calcium hydroxide when it meets water and gives off a lot of heat in the process. "Kalkwasser" and "limewater" properly describe the solution, not the powder, so a tub labeled kalkwasser actually contains lime or quicklime. Food-grade pickling lime is normally calcium hydroxide and works, while garden or agricultural "lime" is often ground limestone (calcium carbonate), which will not dissolve and should not be used.

Always mix kalk in fresh water. Stirring lime into saltwater produces a mess of precipitated magnesium and calcium solids instead of usable limewater.

How much calcium and alkalinity kalk adds (with the math)

Calcium hydroxide is only slightly soluble. In the National Bureau of Standards study that made saturated calcium hydroxide a reference pH standard, a saturated solution at 25 °C held about 0.0203 mol/L and read pH 12.45. Colder water dissolves a little more and warmer water a little less.

Calcium: 0.0203 mol/L × 40.08 g/mol ≈ 0.81 g/L ≈ 810 mg/L (ppm). Randy Holmes-Farley gives about 808 ppm.
Alkalinity: each Ca(OH)₂ releases two hydroxide ions, each worth 1 meq, so 2 × 20.3 mmol/L ≈ 40.6 meq/L (Holmes-Farley: about 40.8 meq/L). At 2.8 dKH per meq/L that is about 114 dKH.
Ratio: 808 ppm ÷ 40.8 meq/L ≈ 19.8 ppm calcium per meq/L of alkalinity, which matches the roughly 18–20 ppm per meq/L that corals and coralline algae consume.

That balance is why kalk sits alongside two-part and calcium reactors as a "balanced" supplement. Recipe 1 two-part, for comparison, adds about 1.4 dKH (0.5 meq/L) and 10 ppm calcium per mL per US gallon (its alkalinity part holds about 1,900 meq/L and its calcium part about 37,300 ppm), which is also about 20 ppm per meq/L. Kalk's hydroxide does not stay hydroxide for long: it reacts with dissolved carbon dioxide to form bicarbonate and carbonate, so corals end up seeing ordinary carbonate alkalinity.

Per dose

One liter of saturated kalk in 100 US gallons (378.5 L) of water adds 40.8 ÷ 378.5 ≈ 0.108 meq/L, or about 0.30 dKH, and 808 ÷ 378.5 ≈ 2.1 ppm calcium. Scaling that up gives the handiest rule: every 1% of the system's volume added as saturated kalk raises alkalinity about 1.1 dKH and calcium about 8 ppm.

Treat these as ceilings. Holmes-Farley measured his own working reservoir at about 7 mS/cm against about 10.3 mS/cm for saturated limewater, roughly two-thirds strength, and still met his tank's needs. Your real numbers come from testing the tank, not from the table. Use the tank volume calculator to get the true water volume (display plus sump, minus rock and sand) before you do any of this math.

Why kalk raises pH and consumes CO₂

Hydroxide grabs carbon dioxide from the water and from the air at the surface and turns it into bicarbonate and carbonate. Removing CO₂ and adding alkalinity both raise pH, so kalk gives the largest pH boost per unit of alkalinity of any common additive. That is why Holmes-Farley calls it the best supplement for raising pH, especially in homes with high indoor CO₂ or on tanks running a calcium reactor, whose CO₂-rich effluent drags pH down. See our reef tank pH guide for the other causes of low pH, and the ocean acidification and calcification guide for why pH matters to corals.

The same effect is the risk. Higher pH shifts more bicarbonate into carbonate, and each 0.3 pH unit rise has about the same effect on calcium carbonate supersaturation as doubling calcium or alkalinity. Push pH too high and calcium carbonate starts falling out of solution onto pumps and heaters, or all at once as a white "snowstorm."

The real limit: evaporation

Because kalk is normally dosed as top-off water, you can only add as much as evaporates. The math above turns that into a simple check:

Evaporation needed (% of volume per day) ≈ daily alkalinity consumption in dKH ÷ 1.14
Example: 100 US gallons using 1 dKH/day needs about (1 ÷ 2.8) × 378.5 L ÷ 40.8 ≈ 3.3 L (0.88 gal) of saturated kalk per day.

Holmes-Farley's demand bands for a 100-gallon tank show where kalk runs out: a light load of 50 meq/day equals replacing about 0.3% of the volume with saturated kalk daily, a medium load of 150 meq/day about 1%, and a heavy load of 600 meq/day about 4%. Evaporating 4% of a system's water every day is a tall order, which is why Holmes-Farley says kalk alone may not keep up with tanks at the high end of demand. Evaporation also falls in cool or humid weather while demand does not, which is a classic source of alkalinity swings.

Ways to stretch it:

• More evaporation. A fan over the sump raises evaporation and therefore how much kalk goes in.
• Vinegar in the kalk. Distilled white vinegar lets more lime dissolve. Holmes-Farley's starting point is about 3 level teaspoons of lime and 45 mL of vinegar per gallon of limewater. Downsides: the acetate feeds bacteria, part of the measured alkalinity may briefly be acetate, the mix is still very high in pH, and it is awkward to run through a reactor.
• Top up with two-part. The simplest fix is to let kalk carry part of the load and dose the rest with two-part (worked example below).

One genuine advantage: kalk brings no leftover sodium or chloride along with its calcium and alkalinity, so Holmes-Farley notes it does not raise salinity over time the way long-term two-part dosing slowly can.

How to mix kalk

1. Start with RO/DI water in a dedicated, lidded container.
2. Add the powder to the water (never a little water onto a pile of quicklime, which can boil). Adding more than about 1.5 g per liter is fine; the excess just sits on the bottom and keeps the solution saturated.
3. Stir or shake, cover, and let it settle for a few hours to overnight. Dose the clear liquid, not the milky part.
4. Leave the sediment in place between batches. It traps impurities such as copper and some phosphate. Throw it out once it discolors or after 6–12 months.
5. Ignore the thin crust that forms on top. It is calcium carbonate from air contact and actually shields the liquid below. A covered, still reservoir loses little strength over a few weeks, but bubbling air through limewater ruins it quickly.
6. Do not add magnesium, calcium, or alkalinity products to the kalk. Magnesium precipitates as magnesium hydroxide, extra calcium stops the lime dissolving, and alkalinity supplements precipitate as calcium carbonate.

Dosing methods

Through the auto top-off (most common)

Clear kalk replaces evaporated water through a slow pump controlled by a float switch on the ATO. Use a genuinely slow pump. Holmes-Farley warns that powerheads used this way often push too much in before the float shuts them off. Size the reservoir, or the pump's run-time limit, so that a stuck switch cannot empty more than a small fraction of tank volume, and consider a pH monitor with an alarm. Hardware options are in our ATO buyer's guide.

Slow dosing on a schedule, such as overnight

A dosing pump or dripper can spread kalk across many hours instead of tying it to the float switch. When the goal is pH support, Holmes-Farley suggests timing kalk for night and early morning, when pH is naturally lowest, or running it from a pH controller only when pH dips below about 7.8. That works well alongside a calcium reactor, where the kalk is there mainly to soak up excess CO₂.

Kalk stirrers and reactors (Nilsen-style)

A kalk reactor is a small chamber holding solid lime. Top-off water flows through it on its way to the sump, and a magnetic stirrer mixes the lime and water several times a day. Reactors save space and need refilling about as often as a large reservoir (roughly every few weeks). They cannot deliver more calcium or alkalinity than saturated kalk from a still container, they can struggle to stay fully saturated, and vinegar is hard to use with them. Never load a reactor with quicklime and a small amount of water: the heat has melted and even ruptured reactors.

Small all-at-once doses

Pouring kalk in is only reasonable for tiny amounts. Holmes-Farley's figures: adding 0.25% of the tank volume at once (about 1 L per 100 gallons) raises pH by about 0.1–0.2 units, while 1.25% raises it 0.6–0.7 units, which is far too much. He puts the outer limit for fast additions at about 0.2 meq/L (0.56 dKH) of alkalinity, roughly 0.5% of the volume. Keep single pours near the 0.25% end, only when the tank starts below pH 8.4, and pour into a high-flow area of the sump away from animals. Do not dose undissolved slurry into the display: solid particles can harm animals and trigger precipitation.

Safety: pH spikes, snowstorms, and caustic handling

Protect the tank

Holmes-Farley treats pH 7.8 to 8.5 as an acceptable reef range. In one of his experiments, adding hydroxide equal to 1.4 dKH of alkalinity (about 10 ppm calcium as kalk) in one go took pH from 8.10 straight to 8.76, and it only settled back to 8.33 after the water pulled in more CO₂ from the air. A failed ATO or a dumped jug does the same thing on a larger scale, which is why slow delivery and a capped reservoir matter.

If you overdose:

• pH 8.5 or lower: usually nothing is needed. A milky calcium carbonate cloud tends to clear in a few days without a water change. Do not test calcium or alkalinity while the water is cloudy, because suspended solids read falsely high.
• pH above 8.5: stop the kalk, add aeration (the gentlest fix), and bring pH down only to about 8.5. Plain soda water adds CO₂ (Holmes-Farley's figure is about 6 mL per gallon of tank water for roughly a 0.3 unit drop), or distilled white vinegar (about 1 mL per gallon for a similar initial drop, with a further drop and some oxygen use as bacteria consume it). Add either slowly into high flow in the sump. Stronger acids are a last resort for people who can monitor pH in real time.

Protect yourself

Calcium hydroxide irritates and can burn the eyes and skin, and its dust irritates the airways (NIOSH). Wear eye protection and gloves when mixing, avoid breathing the powder, and rinse any skin contact with plenty of water. If kalk gets in your eyes, flush them immediately and get medical help. Keep the powder sealed, because it absorbs CO₂ from the air and slowly turns into useless calcium carbonate, and store it out of reach of children and pets.

Combining kalk with two-part or a reactor

Because kalk, two-part, and calcium reactors are all balanced, they combine well. A common setup is kalk through the ATO covering the base load, with two-part on a doser making up the difference and turned up in seasons when evaporation drops. Keep them separate: never pour two-part into the kalk reservoir, and dose them at different points in the sump. If kalk keeps pH at 8.4 or above, Holmes-Farley recommends a two-part whose alkalinity part is plain (unbaked) baking soda, which slightly lowers pH, rather than a baked, carbonate-based one such as Recipe 1. Setup details are in our two-part dosing guide and how to raise alkalinity guide.

A note on magnesium

Settled kalk delivers almost no magnesium. Magnesium hydroxide is insoluble at kalk's pH, and Holmes-Farley measured only about 0.017 ppm magnesium alongside 610 ppm calcium in his limewater. Corals and coralline algae still lock some magnesium into their skeletons, so a tank relying on kalk slowly loses it. In his model, replacing 1% of the volume with kalk every day and adding no other magnesium takes magnesium from 1280 to about 1206 ppm over a year (to about 1131 ppm at 2% per day). Water changes cover much of that; otherwise dose magnesium separately, never in the kalk. See our magnesium guide and the Big 3 guide.

Testing cadence

This is the routine we suggest when running kalk:

• First 1–2 weeks, or after any change: alkalinity daily at the same time of day, calcium every few days, and pH at the end of the light period, when it normally peaks (or right after the kalk runs, if you dose at night).
• Once stable: alkalinity two or three times a week, calcium weekly, and magnesium every few weeks.
• Whenever evaporation changes (heating season, a new fan, a lid, or a heat wave), go back to daily alkalinity checks for a few days.

A calibrated pH meter or monitor is worth having with kalk. Our reef testing schedule covers the rest of the routine.

Worked example

A tank holds 75 US gallons of actual water (about 284 L). After stopping all dosing for 24 hours, alkalinity fell 1.8 dKH. The ATO reservoir shows the system evaporates about 0.75 gallons (2.8 L) a day.

1. Evaporation as a share of volume: 0.75 ÷ 75 = 1% per day.
2. What kalk can supply: 1% × 1.14 ≈ 1.1 dKH and about 8 ppm calcium per day, if the kalk is saturated.
3. Shortfall: 1.8 − 1.14 ≈ 0.66 dKH per day.
4. Cover it with two-part: at Recipe 1 strength (1.4 dKH per mL per gallon), 0.66 ÷ 1.4 × 75 ≈ 35 mL of each part per day, split into several doses. Commercial products differ, so check the label.
5. Kalk alone would need 1.8 ÷ 1.14 ≈ 1.6% of the volume per day, or about 1.2 gallons of evaporation. This tank does not evaporate that much, so it needs the top-up.
6. Retest after 3–4 days. If the kalk is weaker than saturated, alkalinity will still drift down, so raise the two-part about 10% at a time.

If alkalinity has already slipped to 7.6 dKH and the target is 8.3, correct it first with a one-off dose (open this example in the calculator; it works out to about 6 g of sodium bicarbonate), and pause kalk that day if pH is already near 8.4. For other volumes, start from the prefilled alkalinity calculator and change the numbers. If you are using water changes to reset calcium, alkalinity, and magnesium, the water change calculator shows how much each change moves them.

Common kalk mistakes

• Dumping a jug of kalk into the display to "catch up."
• Using a powerhead or fast pump as the kalk top-off.
• Mixing kalk in saltwater, or adding magnesium or two-part to the reservoir.
• Dosing the milky part of a fresh batch.
• Assuming the reservoir is saturated, then chasing a slow alkalinity drift with bigger kalk doses.
• Forgetting magnesium over months of kalk-only dosing.

FAQ

How much does kalkwasser raise alkalinity? Saturated kalkwasser carries about 40.8 meq/L of alkalinity and about 808 ppm of calcium. Replacing 1% of your total water volume with it adds roughly 1.1 dKH and 8 ppm calcium. One liter in 100 US gallons adds about 0.3 dKH and 2 ppm calcium. Real kalk is often weaker than saturated, so confirm with tests.

Can kalkwasser be my only calcium and alkalinity supplement? Only if your daily evaporation is large enough to carry your demand. As a rule of thumb, each 1 dKH per day of consumption needs about 0.9% of the system volume per day in saturated kalk top-off. High-demand SPS tanks usually outrun that and add two-part or a calcium reactor.

What pH is too high when dosing kalk? Randy Holmes-Farley treats pH 7.8 to 8.5 as acceptable for reef aquaria. If kalk pushes the tank above about 8.5, slow or pause the kalk, add aeration, and bring pH back down no lower than 8.5 using the gentle steps in this guide.

Can I mix kalk with saltwater or other supplements? No. Make kalk only in fresh RO/DI water. Mixing lime into saltwater precipitates magnesium and calcium solids, and adding magnesium, calcium, or alkalinity supplements to the kalk reservoir makes them precipitate or stops the lime dissolving.

What should I do after a kalk overdose or snowstorm? Check pH first. If it is 8.5 or lower, the white calcium carbonate cloud usually clears by itself over a few days, and you should wait until the water is clear before testing calcium and alkalinity. If pH is above 8.5, increase aeration and lower it gradually to about 8.5 with carbon dioxide (plain soda water) or distilled white vinegar added slowly into high flow.

References

• Randy Holmes-Farley, "What Your Grandmother Never Told You About Lime," Reefkeeping Magazine, January 2005 (saturation values, dosing methods, vinegar, impurities, magnesium model, overdose steps, and safety).
• Randy Holmes-Farley, "How to Select a Calcium and Alkalinity Supplementation Scheme," Advanced Aquarist, February 2003 (archived at Reefs.com) (demand bands, evaporation limit, snowstorms, and Nilsen reactors).
• Randy Holmes-Farley, "A Simplified Guide to the Relationship Between Calcium, Alkalinity, Magnesium and pH," Reefkeeping Magazine, June 2006 (18–20 ppm calcium per meq/L, and the pH effect on precipitation).
• Randy Holmes-Farley, "Solutions to pH Problems," Advanced Aquarist, June 2002 (archived at Reefs.com) (night-time and pH-controller kalk dosing).
• Randy Holmes-Farley, "pH and the Reef Aquarium," Reef2Reef, 2015 (7.8–8.5 range, the 8.10 to 8.76 hydroxide test, and soda water and vinegar doses).
• Randy Holmes-Farley, "An Improved Do-it-Yourself Two-Part Calcium and Alkalinity Supplement System," Reefkeeping Magazine, February 2006 (Recipe 1 and Recipe 2 strengths, and which to pair with limewater).
• Roger G. Bates, Vincent E. Bower, and Edgar R. Smith, "Calcium Hydroxide as a Highly Alkaline pH Standard," Journal of Research of the National Bureau of Standards 56(6): 305–312, 1956 (solubility about 0.0203 mol/L and pH 12.45 at 25 °C).
• NIOSH, Pocket Guide to Chemical Hazards: Calcium hydroxide (eye, skin, and respiratory hazards, and first aid).

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