Reef dosing calculator
Enter your test result, the level you want and your water volume, and see exactly how much baking soda, soda ash, calcium chloride or magnesium to add, split into daily doses that do not move the tank too fast.
What do you want to raise?
Usual reef range: 7 to 11 dKH.
Include the sump, less the space taken by rock and sand. Work out your water volume.
Gentle on pH, so the usual choice for a correction.
Enter your test results and water volume to see how much to add.
How the amount is worked out
The products here are plain chemicals, so their strength is fixed by chemistry rather than taken from a label. Baking soda (sodium bicarbonate) adds one unit of alkalinity per molecule and soda ash (sodium carbonate) adds two, which is why about 1.9 g of soda ash does the work of 3 g of baking soda. Raising a 100-litre tank by 1 dKH takes 3.0 g of baking soda.
Calcium and magnesium work the same way, from how much of each salt is calcium or magnesium. Raising 100 litres by 20 ppm of calcium takes 7.3 g of calcium chloride dihydrate, and raising it by 100 ppm of magnesium takes 84 g of magnesium chloride hexahydrate. If you use a bottled product, pick the last option and enter the strength from its label.
How fast is safe?
Corals react to how quickly the water changes, not just to where it ends up. The calculator splits any correction bigger than a cautious day's change into equal daily doses: no more than 1 dKH of alkalinity, 20 ppm of calcium or 100 ppm of magnesium a day. Guidance varies and some keepers move faster, but a slower correction costs nothing but time.
Retest before each dose. Corals and coralline algae use alkalinity and calcium all the time, so the tank will not stand still while you correct it. If a level keeps falling, that is consumption, and the answer is a regular daily dose rather than repeated corrections.
What to aim for
Most reef tanks do well with alkalinity of 7 to 11 dKH, calcium of 380 to 450 ppm and magnesium of 1,250 to 1,400 ppm. Stability matters more than any exact figure: a tank held steady at 8 dKH is better off than one swinging between 7 and 10.
Check magnesium first when alkalinity and calcium will not stay up. When magnesium runs low, calcium and alkalinity tend to precipitate out together, and dosing more of them does not fix it.
Choosing and preparing the chemicals
- Use pure products with nothing added. Supermarket baking soda is fine if it is pure sodium bicarbonate. Avoid calcium chloride sold for de-icing, which can contain impurities.
- Check which form of calcium chloride you have. The anhydrous form is about a third stronger than the dihydrate, so choosing the wrong one overdoses.
- Soda ash can be made by baking baking soda in the oven at 150 °C for an hour. It raises pH as well as alkalinity, which helps a tank that runs low on pH but means adding it slowly.
- For regular magnesium dosing, a mix of magnesium chloride and magnesium sulfate keeps the balance of other ions closer to seawater than either one alone.
- Dissolve each dose in RO/DI water before adding it, and never mix the alkalinity and calcium solutions together.
Common questions
How much baking soda raises alkalinity by 1 dKH?
About 3 g per 100 litres, or roughly 11.4 g per 100 US gallons. Soda ash needs about 1.9 g per 100 litres for the same rise.
How much calcium chloride raises calcium by 10 ppm?
About 3.7 g of calcium chloride dihydrate per 100 litres, or 2.8 g of the anhydrous form.
Can I raise alkalinity quickly if it is very low?
It is safer not to. Large, fast swings in alkalinity can stress corals. The calculator splits the correction into daily doses of no more than 1 dKH, and you should retest before each one.
Why does magnesium matter for calcium and alkalinity?
Low magnesium makes calcium and alkalinity hard to keep up, because they precipitate out of the water together. Bring magnesium into range first, then correct the other two.
Should I include the sump in the volume?
Yes. Dose for all the water in the system, the display tank and the sump together, less the space taken by rock and sand.
