Why CO2 Matters in a Planted Tank
Carbon dioxide is the primary carbon source for aquatic plant photosynthesis. In the open air, CO2 is abundant at roughly 400 ppm. In a closed aquarium at equilibrium with the atmosphere, CO2 dissolves to approximately 3-4 ppm — the absolute minimum for basic plant survival, insufficient for active growth. In a densely planted, well-lit aquarium, plants can deplete this ambient CO2 to near zero within hours of the lights turning on, at which point photosynthesis stops entirely.
Adding CO2 to your aquarium — raising dissolved CO2 to 20-30 ppm — unleashes exponential growth in your plants. Crypts that grew one leaf per month may produce one per week. Stem plants that struggled to stay green become lush and dense. More importantly, faster-growing plants outcompete algae more aggressively, resulting in a cleaner, more stable tank.
CO2 supplementation is not mandatory for a planted tank — the species-selection article covers many plants that grow well without it. But for medium and high-demand species, especially carpeting plants, stem plants with intense red coloration, and aquascaping competitive species, CO2 is essential rather than optional.
Pressurized CO2 — The Gold Standard
A pressurized CO2 system consists of a CO2 cylinder (usually 800g or 2kg), a regulator with a solenoid valve, tubing, a diffuser, and a drop checker. The regulator reduces cylinder pressure to a safe working pressure and meters the flow rate in bubbles per second (BPS). The solenoid valve connects to a timer and automatically cuts CO2 at night when plants are not photosynthesizing — saving gas and preventing pH crashes.
CO2 cylinders are available at welding supply shops and some aquarium specialty stores in Phnom Penh. Cost per refill for an 800g cylinder is approximately $5-8 USD, lasting 4-8 weeks depending on tank size and injection rate. The initial investment ($40-80 for regulator + fittings) pays for itself rapidly compared to ongoing liquid carbon costs.
Inject CO2 1 hour before lights turn on so the water is pre-charged when photosynthesis begins. Stop injection 1-2 hours before lights go off — residual CO2 will persist in solution while fish need oxygen overnight. A solenoid timer automates this entirely.
Bubble count starting points: 1 BPS per 50 liters of tank volume for low-tech light, 2 BPS for medium light, 3-4 BPS for high-intensity (T5HO or high-power LED). Adjust based on drop checker readings over 1-2 weeks.
- ✦Never transport a pressurized CO2 cylinder in an enclosed vehicle — use pickup beds or rooftop carriers
- ✦Check all connections monthly for leaks using soapy water — bubbles indicate a leak
- ✦A 2kg cylinder is more economical per gram of CO2 than an 800g cylinder
- ✦Inline diffusers installed in canister filter outflow dissolve CO2 more efficiently than in-tank glass diffusers
DIY Yeast CO2 — Budget Option
A DIY CO2 system uses the fermentation of yeast and sugar to produce CO2. The classic recipe: 250g sugar, 1/4 teaspoon dry yeast (bread yeast), 1/2 teaspoon baking soda (pH buffer), water to fill a 1.5-liter bottle. Shake gently, connect tubing through the cap to a diffuser, and CO2 production begins within 30-60 minutes.
DIY CO2 has zero upfront cost if you repurpose a plastic bottle. The mixture produces CO2 for 2-4 weeks before the sugar exhausts and must be remixed. The main limitation is inconsistency — production rate varies with temperature (higher ambient temperature = faster fermentation = more CO2) and the mixture weakens as it ages. You cannot control output precisely, and you cannot add a solenoid to stop production overnight.
DIY CO2 is appropriate for small tanks (under 60 liters) with medium-light plants when pressurized CO2 is not in the budget. For competitive aquascaping or demanding species, invest in pressurized CO2 — the inconsistency of DIY is a significant limitation.
Liquid Carbon (Excel/Glutaraldehyde) — The Chemical Alternative
Liquid carbon products (Seachem Excel, Easy-Life EasyCarbo, and generics) contain glutaraldehyde, which plants can use as a carbon source through a different biochemical pathway than CO2. They do not replace gaseous CO2 injection for demanding species, but they provide meaningful carbon supplementation for low-tech tanks with easy plants.
Liquid carbon has an important secondary function: at overdose levels (2-3x the stated dose applied directly to algae via pipette or syringe), it is an effective spot treatment for stubborn algae species like black beard algae, staghorn algae, and green spot algae. Apply directly onto the algae with lights off, wait 5 minutes, then resume normal flow. The algae turns red and dies within a few days.
Dose liquid carbon daily — it degrades within 24 hours. Remove activated carbon from filters when using liquid carbon products. Invertebrates (shrimp, snails) are sensitive to glutaraldehyde at high doses — never exceed the manufacturer's recommended rate in shrimp tanks.
The Drop Checker — Reading Your CO2 Level
A drop checker is a small glass bulb filled with a pH-sensitive indicator solution (usually 4 dKH reference water + bromothymol blue indicator). It hangs in your tank and changes color based on dissolved CO2 levels: blue = CO2 too low, green = ideal (25-30 ppm), yellow = CO2 too high (dangerous for fish). Check the drop checker 1 hour after your CO2 turns on during the light period.
In Cambodia's warm water (28-30°C), CO2 solubility is lower than in cooler tanks, meaning you may need to inject at a higher bubble rate to achieve the same dissolved CO2 level compared to a 24°C tank. Adjust BPS upward and recheck the drop checker after 48 hours of stable running before making another adjustment.
Fish behavior is the fastest CO2 alarm. Fish gasping at the surface, clustering near the water outlet, or lying on the substrate are signs of CO2 overdose and oxygen depletion. Immediately open windows, add aeration, and reduce CO2 injection rate. Do not overdose CO2 at night when plants are consuming oxygen rather than producing it.