Find out how much saturated kalkwasser your reef needs each day, how much calcium and alkalinity each liter adds, and whether your evaporation is big enough to carry it. If it isn't, the calculator shows the shortfall to cover with two-part.
How the math works
Saturated limewater at about 25 °C holds roughly 0.0203 mol/L of calcium hydroxide (Bates, Bower, and Smith (1956), "Calcium Hydroxide as a Highly Alkaline pH Standard," Journal of Research of the National Bureau of Standards 56(6): 305–312). That works out to about 808 ppm calcium and 40.8 meq/L alkalinity, or about 114 dKH, the figures Holmes-Farley gives in Randy Holmes-Farley, "What Your Grandmother Never Told You About Lime," Reefkeeping Magazine, January 2005. Diluted into your system:
dKH added per liter of kalk = 40.8 × 2.8 ÷ system liters
Calcium added per liter (ppm) = 808 ÷ system liters
Kalk needed per day (L) = daily dKH use × system liters ÷ 114
A handy shortcut: every 1% of your system's volume added as saturated kalk raises alkalinity about 1.1 dKH and calcium about 8 ppm. Because kalk normally goes in as top-off water, evaporation sets the ceiling.
Saturated kalk needed per day
| System volume | 0.5 dKH/day | 1 dKH/day | 2 dKH/day |
|---|---|---|---|
| 20 gal (76 L) | 0.3 L (0.09 gal) | 0.7 L (0.18 gal) | 1.3 L (0.35 gal) |
| 40 gal (151 L) | 0.7 L (0.18 gal) | 1.3 L (0.35 gal) | 2.7 L (0.70 gal) |
| 75 gal (284 L) | 1.2 L (0.33 gal) | 2.5 L (0.66 gal) | 5.0 L (1.31 gal) |
| 100 gal (379 L) | 1.7 L (0.44 gal) | 3.3 L (0.88 gal) | 6.6 L (1.75 gal) |
| 150 gal (568 L) | 2.5 L (0.66 gal) | 5.0 L (1.31 gal) | 9.9 L (2.63 gal) |
| 200 gal (757 L) | 3.3 L (0.88 gal) | 6.6 L (1.75 gal) | 13.3 L (3.50 gal) |
Computed from the formula above for fully saturated kalk. Your evaporation has to be at least this large for kalk alone to keep up.
Dose it safely
Kalk raises pH quickly. Add it slowly (through an auto top-off, a drip, or a reactor), keep tank pH at or below about 8.5, and retest alkalinity daily while you dial it in. Mix it only in fresh RO/DI water and handle dry lime with gloves and eye protection. Our kalkwasser dosing guide covers mixing, reactors, overdoses, and pairing kalk with two-part, and the dosing calculator handles one-time corrections.
FAQ
How much kalkwasser should I dose per day? Only as much as your tank uses. Saturated kalk carries about 40.8 meq/L of alkalinity (about 114 dKH), so a 100-gallon system using 1 dKH per day needs about 3.3 L (0.88 gal) of it a day. Start below that, add it slowly through the day, and adjust by testing.
Can my evaporation handle kalk alone? Divide your daily alkalinity use in dKH by 1.14 to get the share of your water that has to evaporate, and be replaced with kalk, each day. A tank using 1.8 dKH a day needs about 1.6%. If it only evaporates 1%, kalk covers about 1.1 dKH a day and the rest needs two-part or a calcium reactor.
Why does my kalk raise alkalinity less than the calculator says? The numbers assume fully saturated kalk. Lime that hasn't fully dissolved, a reactor that doesn't reach saturation, or limewater left open to air (carbon dioxide turns some of it into calcium carbonate) all deliver less. Lower the strength field until the prediction matches your tests.