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CO₂ from pH drop (+- 10%)

In planted aquariums, a pH drop is the difference between fully degassed pH (no added CO₂) and the pH measured during CO₂ injection. Each 1.0 pH drop indicates roughly 10× more dissolved CO₂. A common rule-of-thumb is that a 1.0 drop ≈ 30 ppm CO₂.

This method is an estimate. Actual ppm depends on your carbonate hardness (KH), temperature, and measurement accuracy.

CO₂ Concentration Calculator

Estimate CO₂ levels in your aquarium based on pH and kH (carbonate hardness).

How to calculate the pH drop in an aquarium?

1) Brief explanation

CO₂ lowers the pH because it forms carbonic acid in water. If you completely degas a sample of your aquarium water, the pH rises again. The difference between both pH measurements is your pH drop.

Example: Tank pH = 5.0 and pH after degassing = 6.0 → ΔpH = 1.0 → roughly ~30 ppm CO₂ (often considered "ideal" for planted aquariums).

Important: When does this work reliably?

The carbonate hardness (KH) must remain unchanged during both measurements.
There should be no strong additional acid/base sources (e.g. active soil effects, large humic acid additions).
You need a clean pH measurement (calibrated, temperature-stable, same sample/same temperature).

2) What you need

pH meter (recommended) or very precise drop/photo tests
2 cups/glasses (for "CO₂ active" and "degassed")
Membrane pump + diffuser or vigorous surface aeration for the sample
Timer (approx. 15–30 minutes)
Optional: Thermometer (temperature equality is important)

Setup tip

Always measure at the same time of day if possible, e.g. 2–3 hours after lights on, when CO₂ has stabilised in the tank.

3) Step-by-step procedure

Step 1 – Measure pH during operation

Measure the pH directly in the aquarium while CO₂ has been running for a while (stable operation). Record the value as pH_active.

Step 2 – Take a water sample

Take a sample from the aquarium (e.g. 200–500 ml) into a clean glass. Ensure the temperature matches the tank as closely as possible.

Step 3 – Fully degas the sample

Aerate the sample vigorously (membrane pump + diffuser) for about 20 minutes (depending on strength, 15–30 minutes), until the pH no longer rises. This is your pH_degassed.

Step 4 – Calculate the pH drop

ΔpH = pH_degassed − pH_active
A ΔpH of 1.0 is commonly used as the target value.

Step 5 – Calculate CO₂

Use the CO₂ from pH drop calculator to calculate your CO₂ level.

Quality check: If you still see a pH increase after another 5 minutes of aeration, the sample was not yet fully degassed.

4) Interpretation: What does your ΔpH mean?

Many aquarists use as a rule of thumb: ΔpH ≈ 1.0 roughly equals ~30 ppm CO₂ (at "typical" KH ranges). This is a practical target range for heavily planted tanks, as long as livestock shows no signs of stress.

Quick reference table

ΔpH	Rough CO₂ estimate	Comment
0.3	low	often too little for demanding plants
0.6	medium	often "getting there"
1.0	high / target range	Target value for high-end planted tanks
1.2+	very high	Risk for fish/shrimp increases, Söchting Oxydator recommended

Note: Exact ppm depends on the carbonate system (KH/alkalinity). The pH drop is primarily a practice-oriented indicator, not a laboratory value. The actual ppm is approximately ±5 ppm, so it is accurate enough for plant cultivation.

CO₂ Concentration Calculator - Tools & Calculators