CO2: The Plant Steroid

In a standard aquarium, plants are limited by one thing: Carbon. Algae has a physiological advantage—it can extract carbon from carbonates in the water much better than complex plants can. By injecting CO2 gas, we level the playing field.

The Photosynthesis Equation

$$ 6CO_2 + 6H_2O + Light \rightarrow C_6H_O_6 (Sugar) + 6O_2 $$

Plants need Carbon Dioxide + Water + Light to make food. If you increase the Light but do not increase the $CO_2$, the plants starve, and algae (which needs less $CO_2$) takes over.

High Light requires High CO2.

Measuring CO2: The Drop Checker

You cannot easily test dissolved CO2 with a liquid kit. Instead, we use a Drop Checker. This is a glass bulb filled with a solution of 4dKH indicator fluid. It changes color based on the pH shift caused by CO2.

• Blue: Not enough CO2 (< 15 ppm). Plants will struggle.
• Green: Optimal (30 ppm). Safe for fish, great for plants.
• Yellow: Too much CO2 (> 45 ppm). DANGER. Fish will gasp and suffocate.

The pH / KH / CO2 Relationship

Injecting $CO_2$ creates Carbonic Acid, which lowers pH. We can calculate the exact ppm of CO2 if we know the pH and KH (assuming no other buffers are present).

The Chart:

• KH 4 + pH 6.6 = ~30 ppm CO2 (Green)
• KH 4 + pH 7.2 = ~7 ppm CO2 (Blue - Low)

Safety First

1. Timer Solenoid: CO2 should turn ON 1 hour before lights and turn OFF 1 hour before lights go out. Plants release CO2 at night; adding more can suffocate fish.
2. Surface Agitation: You still need oxygen! Do not stop your filter ripple. Good flow ensures CO2 is distributed without creating specific death zones.