Plain baking soda (sodium bicarbonate) raises reef tank alkalinity cheaply and safely. It takes about 0.114 grams per US gallon of water to raise alkalinity by 1 dKH, which works out to roughly 11.4 grams per 100 gallons. Dissolve it in RO/DI water, add it slowly to a high-flow area, and don’t raise alkalinity by more than about 1 dKH a day.
How much baking soda raises alkalinity by 1 dKH? #
The number comes straight from the chemistry. One dKH equals 0.357 milliequivalents per liter (meq/L). Each gram of sodium bicarbonate supplies about 11.9 meq, since its molecular weight is 84. A US gallon is 3.785 liters, so a 1 dKH rise in one gallon needs about 1.35 meq, or 0.114 grams.
Use your net water volume, not the tank’s advertised size. Rock, sand and an empty sump section can take 10-20% off the real volume, and dosing for the full number overshoots.
| Net water volume | Baking soda for +1 dKH | For +0.5 dKH |
|---|---|---|
| 20 gal (76 L) | 2.3 g | 1.1 g |
| 40 gal (151 L) | 4.5 g | 2.3 g |
| 75 gal (284 L) | 8.5 g | 4.3 g |
| 100 gal (379 L) | 11.4 g | 5.7 g |
| 150 gal (568 L) | 17.0 g | 8.5 g |
| 200 gal (757 L) | 22.7 g | 11.4 g |
In metric, it’s about 3.0 grams per 100 liters for each dKH. Weigh it on a kitchen scale instead of guessing with spoons. Spoon volumes vary more than you’d expect, and this is a number you don’t want to miss by double.
Should you use baked or plain baking soda? #
Both raise alkalinity. The difference is what they do to pH.
Plain baking soda (sodium bicarbonate) has a pH near that of tank water. It raises alkalinity without pushing pH up, and it can even dip pH briefly. Use it if your pH already runs high, above about 8.3.
Baked baking soda is sodium carbonate. Heating baking soda drives off water and carbon dioxide, and what’s left raises both alkalinity and pH. Per gram it’s also more potent. It takes about 72 mg per gallon per dKH instead of 114 mg. Use it if your pH runs low, which is the more common problem in closed-up homes.
Chemist Randy Holmes-Farley’s DIY two-part recipe describes both versions:
- Recipe #1 (baked): Bake 2 1/4 cups (594 g) of baking soda at 300°F for an hour. Measure it before baking, not after. Then dissolve it in enough RO/DI water to make one gallon. The solution holds about 1,900 meq/L, or 5,300 dKH.
- Recipe #2 (unbaked): Dissolve 297 g (about 1 1/8 cups) of baking soda in enough RO/DI water to make one gallon. The solution holds about 950 meq/L, or 2,660 dKH.
Randy notes that adding about 0.5 meq/L of alkalinity with the baked version raises pH by about 0.3 units right away. That’s useful for low pH, and a reason to go slowly if your pH is already high. More on that in how to raise pH in a reef tank.
How do you dose baking soda into a reef tank? #
- Test alkalinity twice. If both readings agree, you have a real number. If they don’t, see the test kit section below.
- Pick a target. Most reef tanks aim for 7-11 dKH. Raise toward the level you plan to hold, not the top of the range.
- Work out the dose. Multiply the dKH you want to add by the grams per dKH for your net volume. If the gap is more than 1 dKH, split it across days.
- Dissolve it fully. Stir the powder into a cup or two of RO/DI water until it’s clear. Undissolved grains landing on rock or coral can burn tissue.
- Add it slowly to high flow. Pour it into the sump near the return pump or into a powerhead’s path over several minutes. Avoid dosing near where you add calcium, because the two can precipitate as white “snow.”
- Retest. Test again a few hours later, once the water has mixed, or the next morning. Compare the result with what you expected.
- Repeat if needed. Continue the next day until you reach the target.
What about a mixed solution or a commercial product? #
If you keep a jug of mixed solution for daily dosing, the volume depends entirely on its strength:
| Product | Rate stated by maker | Amount for +1 dKH in 100 gal |
|---|---|---|
| Randy’s Recipe #2 (unbaked, 950 meq/L) | Calculated | About 142 ml |
| Randy’s Recipe #1 (baked, 1,900 meq/L) | Calculated | About 71 ml |
| Red Sea Reef Foundation B (liquid) | 1 ml per 100 L raises 0.1 dKH | About 38 ml |
| ESV B-Ionic Alkalinity | 1 ml per gallon raises 2.07 dKH | About 48 ml |
Always follow the label on the bottle you own, and never reuse one product’s dose for another. Concentrations differ severalfold. ESV, for example, says never to add more than 1 ml per gallon of its alkalinity part in 24 hours, and to keep pH below 8.45.
How fast can you raise alkalinity safely? #
A common rule among reef keepers is no more than about 1 dKH per day. Corals respond to the rate of change as much as to the final number, and a fast jump is a classic trigger for SPS tissue loss, especially in tanks that have been running low for a while. If you’re at 6 dKH and want 8.5, plan on three days.
Fast dosing also risks precipitation. If alkalinity, calcium and pH all go up at once in one spot, calcium carbonate can form on heaters, pumps and in the water column. That wastes both supplements and can leave you with lower readings than you started with.
Why did my alkalinity drop in the first place? #
Dosing baking soda fixes the number, but if it keeps falling you need a daily maintenance dose. Why is my alkalinity dropping covers the causes and how to measure daily consumption. Keep in mind that baking soda only supplies alkalinity. If you dose it for months with nothing else, calcium will fall as corals use both. Pair it with a calcium part, as covered in how to raise calcium in a reef tank.
Can you trust the test result? #
Before dosing anything, make sure the low reading is real. Alkalinity tests vary. Hanna’s HI772 checker reads 0-20 dKH with an accuracy of ±0.3 dKH ±5% of the reading, so a 10.0 display could mean anything from about 9.2 to 10.8. Salifert’s titration kit measures in steps of 0.1 meq/L, about 0.3 dKH. Old reagents are a frequent cause of readings that look low. We compare the two in Hanna checker vs Salifert for alkalinity.
How do you track alkalinity doses? #
The best defense against overshooting is a written record: what you dosed, when, and what the next test showed. Fish Tank Tracker logs alkalinity in dKH next to calcium, magnesium and pH. Each test gets a notes field for “8.5 g baked soda, 9 pm,” and the chart draws every reading against an 8-11 dKH band so an overshoot stands out. If your kit reports meq/L, multiply by 2.8 before you log it.
Frequently asked questions #
Can I use Arm & Hammer baking soda in my reef tank? #
Yes, as long as it’s plain baking soda with sodium bicarbonate as the only ingredient. Avoid anything labeled for laundry, deodorizing or with added fragrance. The same goes for washing soda (sodium carbonate): use it only if the label lists nothing else.
How long after dosing baking soda should I test alkalinity? #
Wait until it has fully mixed through the system, at least an hour in most tanks, though many people simply test the next morning. Testing too soon, near the dosing point, will read high.
Does baking soda lower pH in a reef tank? #
Plain baking soda can nudge pH down slightly for a short time, though it raises alkalinity, which supports pH over the longer term. Baked baking soda (sodium carbonate) raises pH immediately. Pick the version that suits the pH your tank already has.
Can I add baking soda directly to the display tank? #
Dissolve it first and add it where the flow is strong, ideally in the sump. Dry powder or a concentrated plume landing on coral can burn tissue, and it can cause local precipitation.
What is the fastest safe way to raise very low alkalinity? #
Raise it in steps of about 1 dKH a day and test between doses. Going from 5 to 9 dKH overnight often does more harm than a few more days at a low but stable level.