How to raise alkalinity in a reef tank

Short answer: to raise alkalinity, dose an alkalinity supplement — baking soda, soda ash, or a commercial buffer — in an amount calculated for your water volume, and raise it no faster than about 1 dKH per day. If you need a bigger jump, split it over several days. Here's how much to use, which product to pick, and how to do it without harming your tank.

How much to add

The amount depends on your net water volume and how far you're raising alkalinity. The cheapest reliable option is plain baking soda: roughly 3 g of baking soda raises 100 litres (26 US gal) by about 1 dKH. Soda ash is stronger — about 1.9 g per 100 litres per dKH — but it also raises pH.

Rather than measure by the teaspoon (which varies a lot), use the exact figure for your tank:

Two things the calculator handles that a rule of thumb won't: it uses your net water volume (rock and sand displace 10–20%, so your tank holds less water than its rated size), and it enforces the safe daily limit.

Baking soda or soda ash?

Both raise alkalinity; the difference is what they do to pH.

• Baking soda (sodium bicarbonate) is roughly pH-neutral. Use it when your pH is fine or on the high side.
• Soda ash (sodium carbonate) raises pH as well as alkalinity. Use it when your pH runs low — common in closed-up homes with high indoor CO₂.
• You can turn baking soda into soda ash by baking it in the oven (about 150 °C / 300 °F for an hour). A cheap way to make the pH-raising version from a box of baking soda.

Commercial buffers (Red Sea, Seachem, Brightwell, two-part systems) work the same way — you're paying for convenience and consistency, not better chemistry. The calculator covers the common ones.

The safe daily limit

Raise alkalinity by no more than about 1.0 dKH per day — and closer to 0.5 dKH/day for an SPS-dominant tank. The limit isn't about alkalinity being toxic; it's that raising it fast stresses coral and pushes supplement impurities in quickly. If you're 3 dKH low, don't fix it in one dose — split it over three days. The calculator does that split for you.

How to dose it safely

• Add it to a high-flow area, slowly. A big dose dumped in one spot spikes alkalinity locally.
• Never dose alkalinity and calcium at the same time or in the same place — together they can precipitate out as a "snowstorm" that strips both. Space them apart.
• If your magnesium is low, correct it first — otherwise the alkalinity you add won't hold. Here's how to raise magnesium.
• Retest after a day. One reading isn't the whole story; look at the trend.

Raising vs. maintaining

Raising alkalinity is a one-time correction. Keeping it there is the real job — and that's about matching your daily dose to how fast your tank consumes alkalinity. If yours keeps drifting down, see why alkalinity drops, and let the app work out your consumption so your dose actually keeps up. Aim for steady, not perfect.

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