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CO2 Injection Rate Calculator

A starting bubble-count estimate by tank size, plus the standard pH/KH/CO2 formula to check your actual dissolved CO2 level once it's running.

Your numbers
Starting bubble rate

Start CO2 before your lights turn on

Start CO2 injection an hour or two before your lights turn on, and shut it off before they turn off — that way CO2 is already at target once photosynthesis ramps up.

The pH/KH/CO2 formula

Dissolved CO2 can be estimated from pH and carbonate hardness (KH) using the standard aquarium chemistry relationship: CO2 (ppm) = 3 × KH (dKH) × 10(7 − pH). This works because CO2 dissolving in water forms carbonic acid, which lowers pH in proportion to how much carbonate buffering (KH) is present to resist that change — so the same CO2 level produces a bigger pH drop in soft, low-KH water than in hard, high-KH water.

Why bubble count is only a starting point

Bubble size varies between diffusers, and CO2 solubility is affected by flow, surface agitation and tank shape, so "bubbles per second" doesn't translate to a fixed ppm across different setups. Use a bubble-count estimate to get roughly in range, then confirm and fine-tune with the pH/KH formula (or a drop checker) rather than trusting bubble count alone.

Target range and fish safety

Most planted tanks target roughly 20–30 ppm dissolved CO2 for good plant growth. Because CO2 injection lowers dissolved oxygen availability somewhat and can stress fish at high levels, many keepers reduce or turn off CO2 injection overnight, when plants aren't photosynthesizing and don't need it anyway, and watch fish for rapid gilling or gasping at the surface as a sign CO2 is too high.

Source: The CO2/pH/KH formula is a standard aquarium chemistry relationship widely published in planted-tank chemistry references, including the classic Rex Grigg CO2 chart still cited across the planted-tank hobby.

Frequently asked questions

What's a drop checker, and how does it relate to this calculator?

A drop checker uses a color-changing indicator fluid calibrated to a reference KH to show roughly whether CO2 is low (blue), in range (green) or high (yellow) at a glance. It's a helpful visual complement to calculating exact ppm from pH and KH, especially for spotting a slow drift over the course of a day.

Why did my pH not change even though I'm injecting CO2?

Very high KH buffers pH strongly against change, so the same CO2 level produces a much smaller pH shift in hard water than in soft water. This is exactly why the formula uses both KH and pH together rather than pH alone.

Should I run CO2 at night?

Most planted-tank keepers turn CO2 off (often on a timer, an hour or so before lights-out) since plants stop consuming CO2 without light, and running it unnecessarily overnight only adds risk for fish without any growth benefit.

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