Aquarium CO2 Injection Rate Calculator

Demanding planted aquariums live or die by their CO2 supply. The ideal dissolved CO2 concentration for most planted tanks is 20–30 ppm (parts per million). Below 15 ppm, many plants struggle. Above 35 ppm, fish and shrimp may show signs of distress.

Estimating CO2 Demand

The baseline CO2 injection rate depends on tank volume and how quickly CO2 is consumed or off-gassed. The starting formula:

CO2 Rate (bubbles per second) = Tank Volume (L) × Plant Factor × Light Factor ÷ 37.85

Where:

• Plant Factor: Low density = 0.5, Medium = 1.0, High = 1.5, Very High (Dutch/Iwagumi) = 2.0
• Light Factor: Low light (< 30 PAR, photosynthetically active radiation) = 0.5, Medium (30–60 PAR) = 1.0, High (60–120 PAR) = 1.5, Very High (> 120 PAR) = 2.0

The 37.85 is not arbitrary. It is the number of litres in a US gallon, so at medium plants and medium light the formula reduces to the rule of thumb every planted-tank guide starts from: one bubble per second per 10 gallons. The factors then move you off that baseline in both directions. The bubble rate calculator works from the same starting point and adds diffuser efficiency on top, so the two pages agree on the same tank.

Worked Example

A 200-liter tank with high plant density and medium lighting: CO2 Rate = 200 × 1.5 × 1.0 ÷ 37.85 = 7.9 bubbles per second

That is a high number, and the page will say so. Past about 5 bubbles a second the bubbles merge into a stream and a bubble counter stops measuring anything. The answer at that point is a better diffuser, not a wider-open needle valve.

CO2 Consumption Reference

(bps = bubbles per second. The Low column uses the 0.5 factors, Medium the 1.0 factors, High the 1.5 factors for both plants and light.)

Cylinder Duration

A standard 5 lb (2.27 kg) CO2 cylinder holds roughly 2,270 grams of gas. A bubble-counter bubble is around 0.2 mL, which at the density of CO2 weighs about 0.36 mg. Running at 3 bps for 8 hours a day:

Daily usage = 3 × 3600 × 8 × 0.00036 g = 31.1 grams/day Cylinder life = 2270 ÷ 31.1 ≈ 73 days

Treat that as an order of magnitude rather than a date. Bubble size varies with the counter’s orifice, the fluid in it and the back pressure, easily by a factor of two between two setups running the same needle valve. Weigh the cylinder instead: a 5 lb cylinder weighs about 5 lb more full than empty, and a luggage scale under the handle tells you exactly where you are.

Always use a drop checker with 4 dKH (degrees of carbonate hardness) reference solution to verify your dissolved CO2 level. The drop checker should turn lime green (about 30 ppm) during the photoperiod. It lags an hour or two behind the tank, so read it in the afternoon, not the moment the solenoid opens.