What it does
Plants use carbon dioxide to build tissue. In most tanks CO₂ runs out long before light does, so adding it lets plants use stronger light without algae taking over.
The kit
A pressurized system uses a CO₂ cylinder, a regulator with a solenoid on a timer, a bubble counter, a diffuser and a drop checker that changes color with CO₂ level. Yeast and citric-acid kits suit small tanks but are less steady.
To see how much CO₂ is in the water, read the drop checker’s color or test pH and KH from the same sample and put them in the CO₂ calculator. Most injected tanks aim for about 20 to 30 ppm.
Keeping fish safe
Too much CO₂ suffocates fish: gasping at the surface is the warning sign. Switch CO₂ on about an hour before the lights and off before they go out, keep the surface moving, and raise CO₂ slowly. Fish gasping at the surface.