Aquarium pH Buffer Calculator

Aquarium pH measures how acidic or alkaline the water is. Most freshwater fish thrive in the range of 6.5–7.8 pH. Marine / reef tanks require a higher pH of 8.1–8.4.

pH scale reference:

What actually sets the pH

Two things do, together: KH (carbonate hardness, the water’s buffering capacity) and the amount of CO2 (carbon dioxide) dissolved in it. They pull in opposite directions. KH pushes pH up, dissolved CO2 pushes it down, and the pH you read on a test kit is where the two settle.

That relationship is not a rule of thumb, it is a fixed chemical equilibrium:

CO2 (mg/L) = 3 × KH (dKH) × 10^(7 - pH)

which is the same equation the Fish Tank CO2 Calculator uses, and it is what a drop checker is really measuring. Rearrange it and you get the part that matters for dosing:

New pH - Old pH = log10(New KH ÷ Old KH)

Read that again, because it is the thing most pH advice gets wrong. pH does not move by a fixed step per teaspoon. It moves with the ratio of the new KH to the old one. Doubling your KH raises pH by 0.30, whatever number you started from. Tripling it raises pH by 0.48. Multiplying it by ten raises pH by exactly 1.0.

The practical consequences are worth spelling out:

• In a soft tank at 1 dKH, one extra degree of hardness doubles the buffer and moves pH by 0.30.
• In a hard tank at 15 dKH, that same extra degree moves it by 0.03, a tenth as much.
• A calculator that quotes one flat “pH per teaspoon” figure is wrong at both ends of that range.

Baking soda (sodium bicarbonate) to raise KH

Baking soda is the cheapest and most predictable way to add carbonate hardness. The dose falls straight out of the definitions, with no rule of thumb involved. One degree of carbonate hardness is 17.848 mg/L expressed as calcium carbonate, which is 0.3567 millimoles of bicarbonate per liter, and sodium bicarbonate weighs 84.01 g/mol:

1 dKH per liter = 29.96 mg of baking soda

A level US teaspoon of baking soda weighs about 4.6 g, so one teaspoon per 10 gallons raises KH by roughly 4 degrees. Weigh it if you can. Teaspoon measures vary by how packed the powder is, and on a small soft-water tank that variation is the difference between a nudge and a shock.

Worked example

A 30 US gallon tank (113.6 L) reading pH 6.8 at 3 dKH, aiming for pH 7.2.

• Dissolved CO2 right now: 3 × 3 × 10^(7 - 6.8) = 14.3 ppm, normal for a lightly planted tank.
• KH needed for pH 7.2 at that same CO2: 3 × 10^(7.2 - 6.8) = 3 × 2.51 = 7.5 dKH.
• Extra hardness required: 4.5 dKH.
• Baking soda: 4.5 × 29.96 mg/L × 113.6 L = 15.4 g, about 3 1/3 level teaspoons.
• At the safe rate of 0.2 pH per day that is two days, so roughly 7.7 g per day.

Now run the same tank at 6 dKH instead of 3. The target becomes 15.1 dKH and the dose jumps to 31 g, almost exactly double, for the identical 0.4 pH move. Start from 8 dKH and the target passes 20, which is harder than most freshwater fish will tolerate, and the honest answer becomes that you cannot get there by buffering at all. That is the ratio law at work, and it is why this number cannot be printed on the back of a box.

When baking soda is the wrong tool

If your CO2 works out above about 20 ppm, low pH is not a buffering problem. The tank is simply carbonated, which is exactly what an injected planted tank is supposed to be. Adding baking soda to “correct” that pushes KH up while the CO2 keeps dragging pH back down, so you chase the reading, end up with rock-hard water, and stress the fish twice over. Turn the CO2 down, add surface agitation, or leave it alone.

Never change pH more than 0.2 per day

Rapid pH changes stress fish severely and can be fatal. Always dissolve the dose in a cup of tank water before adding it, and pour it in near the filter outlet so it disperses instead of landing on a fish. Test pH 24 hours after each adjustment before adding more.

Commercial pH buffers

Products like Seachem Neutral Regulator or API pH Up are more convenient than baking soda but not more precise, and several of them work by adding phosphate rather than carbonate, which does nothing for KH and feeds algae. Read the ingredients before you buy. Follow package directions, because formulations vary widely.

pH drift over the day

In planted tanks, pH rises during the day as plants consume CO2 and falls again at night. A daily swing of 0.3 to 0.5 is normal and not harmful, and it is the ratio law again: a swing of 0.3 just means the CO2 halved between dusk and dawn. Swings larger than 1.0 mean there is almost no KH to buffer them, and that is worth fixing.