How to Raise Alkalinity in a Reef Tank (Safely)

Low alkalinity is the most common chemistry problem in reefkeeping, and also the one most often fixed badly. Raising alkalinity is not hard — the chemistry is simple and the ingredients are cheap — but raising it too fast stresses corals more than the low number ever did. This guide covers the safe targets, the dosing math, and the pace that keeps your corals happy.

What alkalinity should be in a reef tank

The accepted range for reef aquariums is 7–11 dKH (2.5–3.9 meq/L), with most reefers targeting 8–9 dKH. Natural seawater sits around 7 dKH; slightly elevated alkalinity in captive systems supports faster calcification, especially when nutrients are present.

Two rules before you touch a dosing container:

1. Confirm the reading. Test twice with a quality alkalinity titration kit before acting. A single low reading is often a testing error — wrong sample volume, degraded reagent, or a miscounted titration.
2. Pick one target and stay there. Corals tolerate almost any value between 7 and 11 dKH. What they do not tolerate is bouncing between them. Stability beats the "perfect" number every time.

The 1 dKH per day rule

Never raise alkalinity by more than 1 dKH per 24 hours. Corals regulate the chemistry at their growth surface, and a fast alkalinity swing — even in the "right" direction — can cause tissue loss, especially in SPS. Burnt tips on Acropora are the classic symptom of alkalinity moving too fast.

So if your tank reads 6.5 dKH and your target is 8.5 dKH, plan a two-day correction: +1 dKH today, +1 dKH tomorrow, then hold.

What to raise alkalinity with

Sodium bicarbonate (baking soda)

Plain baking soda dissolved in RO/DI water is the gentlest option. It raises alkalinity with only a small, temporary dip-then-recovery in pH, making it safe for tanks that already run at normal pH.

Sodium carbonate (soda ash)

Soda ash is baking soda's stronger sibling — you can make it by baking baking soda at 300°F (150°C) for an hour, which drives off CO2 and water. It raises alkalinity and pH, which makes it the better choice if your tank runs below pH 8.0. Dose it slowly into high flow; concentrated soda ash can locally spike pH and precipitate calcium carbonate.

Commercial two-part

A two-part dosing kit pairs an alkalinity solution (usually carbonate-based) with a balanced calcium solution. It costs more than DIY but removes the guesswork, and it is the natural next step once you are dosing daily.

Kalkwasser

Saturated limewater raises alkalinity and calcium in a balanced ratio and boosts pH, but its concentration is limited by solubility, so it works best as continuous maintenance dosing rather than a correction tool.

The dosing math

Here is the number worth memorizing: about 3 grams of sodium bicarbonate raises 100 liters (26.4 gallons) of water by 1 dKH.

The chemistry: 1 dKH equals 0.357 meq/L. Raising 100 L by 1 dKH requires 35.7 meq of bicarbonate, which is 35.7 mmol of NaHCO3. At a molecular weight of 84 g/mol, that is almost exactly 3.0 g.

Worked example for a 200 L (53 gal) tank at 6.5 dKH with an 8.5 dKH target:

• Total correction needed: 2.0 dKH
• Grams required: 3 g × 2 (for 200 L) × 2 dKH = 12 g total
• Safe schedule: 6 g dissolved in a cup of RO/DI today, 6 g tomorrow

Use your actual water volume — subtract rock and sand displacement, usually 10–15% of the tank's rated volume. Test the next day and adjust; the math gets you close, the test kit gets you exact.

After the correction: find your consumption rate

Raising alkalinity once fixes today. Corals consume alkalinity continuously as they build skeleton, so the level will fall again — and the daily fall rate is the most useful number in reef chemistry. Test at the same time each day for three or four days and note the drop. That daily consumption becomes your daily dose, delivered manually or through a dosing pump.

If you have never measured consumption, read why alkalinity keeps dropping — it walks through the process step by step, and it is the foundation of the whole Reef Chemistry Handbook approach: measure demand first, then supply exactly that.

Mistakes that cause crashes

• Dosing the full correction at once. The 1 dKH/day rule exists because people learned it the hard way.
• Dosing dry powder into the tank. Always pre-dissolve in RO/DI and pour into high flow, away from corals.
• Chasing pH with alkalinity. Alkalinity above 11 dKH does not fix low pH — excess CO2 does that. See the difference between alkalinity and pH before dosing more.
• Ignoring calcium and magnesium. Alkalinity, calcium (400–450 ppm), and magnesium (1250–1350 ppm) work as a system. Low magnesium in particular makes alkalinity hard to hold, because it allows abiotic precipitation of calcium carbonate.
• Trusting one test kit forever. Reagents age. Cross-check a suspicious reading with a fresh kit or a reference standard.

FAQ

How fast can I safely raise alkalinity?

No more than 1 dKH per 24 hours. If you are more than 2 dKH below target, spread the correction over several days. Corals adapt to gradual change remarkably well; they punish speed.

Can I really use baking soda instead of a reef product?

Yes. Food-grade sodium bicarbonate is chemically the same alkalinity source used in many commercial products. The commercial two-part earns its price through convenience, consistent concentration, and the matched calcium part — not through better chemistry.

Will raising alkalinity fix my low pH?

Only slightly, and only within the normal range. If pH sits below 7.9 with alkalinity already at 8–9 dKH, the cause is almost always dissolved CO2 from indoor air, and the fix is gas exchange, not more alkalinity.