CO₂
CO₂ system
The difference between a low-tech planted tank and a high-tech one. Pressurised systems are controllable and safe when set up properly; yeast-based systems are cheap and inherently uncontrollable, which makes them a poor fit for a stocked tank.
Short answer
Equipment that dissolves carbon dioxide into the water for plant growth: a gas source, a regulator, a diffuser and, ideally, a solenoid on a timer.
What it does
- Supplies carbon for photosynthesis
- Lowers pH as a side effect
Strengths and where it fits
Strengths
- Transforms growth rate and colour in demanding plants
- Allows high-light setups to work rather than grow algae
Good fit for
- Planted tanks with medium to high light
- Carpeting plants and demanding stems
Limitations and where it is the wrong choice
Limitations
- Overdosing suffocates fish
- Requires a solenoid and timer to switch off overnight
- Adds ongoing cost and equipment
- Yeast systems cannot be regulated and fluctuate through the day
Poor fit for
- Low-light tanks, where it is unnecessary
- Tanks where nobody will maintain it consistently
Sizing
- Use a drop checker with a 4 °dKH reference solution rather than relying on the pH/KH estimate, which over-reads in tanks with other buffers.
- Start injection an hour or two before lights-on and stop an hour before lights-off.
- Increase the rate gradually over days while watching the fish.