How much baking soda raises alkalinity by 1 dKH?
About 3 grams per 100 liters — 11.4 g per 100 gallons. It isn't a rule of thumb; it falls straight out of the chemistry. One dKH is 0.357 milliequivalents per liter, sodium bicarbonate carries one equivalent per mole, and a mole of it weighs 84.007 g. Soda ash carries two equivalents per mole and weighs less per mole, so it takes 1.89 g for the same job — baking soda needs 1.59× the grams. Everything the calculator does is that arithmetic, scaled to your water and your target.
| System water | Baking soda | Soda ash | Kalkwasser |
|---|---|---|---|
| 10 gal · 38 L | 1.1 g · 0.2 tsp | 0.7 g · 0.1 tsp | 0.33 L |
| 20 gal · 76 L | 2.3 g · 0.5 tsp | 1.4 g · 0.3 tsp | 0.66 L |
| 30 gal · 114 L | 3.4 g · 0.7 tsp | 2.1 g · 0.4 tsp | 0.99 L |
| 40 gal · 151 L | 4.5 g · 1 tsp | 2.9 g · 0.5 tsp | 1.33 L |
| 55 gal · 208 L | 6.2 g · 1.4 tsp | 3.9 g · 0.7 tsp | 1.82 L |
| 75 gal · 284 L | 8.5 g · 1.9 tsp | 5.4 g · 1 tsp | 2.49 L |
| 100 gal · 379 L | 11.4 g · 2.5 tsp | 7.2 g · 1.3 tsp | 3.31 L |
| 125 gal · 473 L | 14.2 g · 3.1 tsp | 9 g · 1.7 tsp | 4.14 L |
| 150 gal · 568 L | 17 g · 3.7 tsp | 10.7 g · 2 tsp | 4.97 L |
| 200 gal · 757 L | 22.7 g · 4.9 tsp | 14.3 g · 2.7 tsp | 6.63 L |
| 300 gal · 1,136 L | 34.1 g · 7.4 tsp | 21.5 g · 4 tsp | 9.94 L |
Per 1 dKH of rise; multiply for your target. Teaspoons are level US teaspoons: baking soda 4.6 g (USDA), dense soda ash ~5.4 g — light, fluffy soda ash is about half that, so weigh it. Kalkwasser is saturated limewater (~0.0204 M). Never more than 1 dKH per day.
Baking soda vs soda ash: which should I use?
Same job, different side effect. Baking soda (sodium bicarbonate) adds a little CO₂ as it dissolves, so pH dips briefly and settles back — safe to add fairly quickly, and the right choice if your pH already runs high. Soda ash (sodium carbonate) pulls CO₂ out of the water instead, so pH rises: good news for the many reefs that run low pH from indoor CO₂, bad news if you dump it in fast — dose it slowly into strong flow, ideally when the lights are off and pH is at its daily low. Neither one adds calcium; that's the other half of a two-part.
| Baking soda | Soda ash | |
|---|---|---|
| Per 1 dKH per 100 L | 3 g | 1.89 g |
| Per 1 dKH per 100 gal | 11.4 g · 2.5 tsp | 7.2 g · 1.3 tsp |
| Effect on pH | lowers pH slightly at first (adds CO₂), then neutral | raises pH — dose slowly, into high flow, ideally when pH is low |
| Best when | pH is high or normal; big single doses | pH runs low; slow daily dosing |
Spread baking soda on a tray and bake it at about 300 °F (150 °C) for an hour: it gives up water and CO₂ and becomes sodium carbonate. Two moles of NaHCO₃ (168 g) become one mole of Na₂CO₃ (106 g), so the pile loses about a third of its weight and gains the pH-raising behavior. That is the entire difference between the two "reef buffers" sold at ten times the price of the grocery-store box.
How fast can I raise alkalinity?
About 1 dKH per day on a stocked reef; corals react to the rate of change more than the number, and the fastest way to lose SPS tissue is a big alkalinity swing. Say a 100 gallon system tests 6.2 and you want 8.5: that's 2.3 dKH, or 26.1 g of baking soda in total — but the calculator splits it into 3 days of 0.77 dKH, 8.7 g each, and you retest before each one, because the tank is also consuming alkalinity while you correct it. Change the "max rise per day" box if your corals are hardy or your reef is bare; the default is the safe one.
Corals build skeleton from calcium and carbonate, and there is roughly twenty times more calcium in seawater than carbonate — so the same day of growth barely dents calcium and visibly drops alkalinity. It moves fastest, it's the easiest to test precisely, and it's the one whose swings hurt. Get alkalinity stable and calcium and magnesium usually follow; the two-part calculator (next in this set) sizes the daily doses that keep it there.
Kalkwasser: how much, and the evaporation ceiling
Kalkwasser — saturated limewater, calcium hydroxide in RO water — adds alkalinity and calcium together in the ratio corals use them, at high pH. It's dilute: about 0.88 L per 100 L of tank per dKH, adding 7.2 ppm of calcium along the way. Because it's dosed as replacement top-off water, it can only supply as much alkalinity as your evaporation lets you drip in: the 100 gallon system above consuming 1 dKH a day would need 3.3 L (0.9 gal) of kalk a day — fine if that's roughly what evaporates, impossible if it isn't. Enter your daily top-off in the calculator and it tells you whether kalk alone can keep up or you need two-part for the rest. Drip it slowly (pH 12.4 in the jug); never pour it.
Bottled buffers and two-part: same chemistry, read the label
Every alkalinity supplement is bicarbonate, carbonate, or a mix, in a bottle or a bag. The calculator's "bottled product" mode takes the strength off the label — "5 mL raises 1 dKH in 25 gallons" or however it's phrased — and scales it to your true volume, which is the part labels can't do for you. If your product only says "1 capful per 25 gallons daily," it's a maintenance product without a stated strength; dose it, test tomorrow, and use the measured change to size the next one.
What should reef alkalinity be? (and the units)
| System | Alkalinity, dKH |
|---|---|
| Natural seawater | 7 |
| SPS, low nutrient (ULNS) | 7–8.5 |
| Mixed reef | 8–9.5 |
| LPS and soft corals | 8–11 |
Consensus hobby ranges. Pick a number inside the range and hold it; stability beats the exact value.
Test kits report in dKH (German degrees), meq/L, or ppm as CaCO₃; the calculator takes any of them. They're fixed multiples of each other: 1 meq/L = 2.8 dKH = 50 ppm.
| dKH | meq/L | ppm CaCO₃ |
|---|---|---|
| 6 | 2.14 | 107 |
| 7 | 2.5 | 125 |
| 8 | 2.86 | 143 |
| 9 | 3.21 | 161 |
| 10 | 3.57 | 179 |
| 11 | 3.93 | 196 |
| 12 | 4.29 | 214 |