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Gas Chemistry
Dissolved CO2
Carbon dioxide dissolved in aquarium water - essential nutrient in planted tanks (20-30 ppm target) but potentially fatal to fish above 40 ppm.
Unit: ppm Code: CO2
📖 Overview
CO2 is the single biggest factor in a lush planted tank. Plants use CO2 for photosynthesis; ambient (non-injected) water rarely exceeds 2-3 ppm. Pressurised CO2 injection raises levels to 20-30 ppm - within 10x more CO2 available for plant growth. But too much CO2 kills fish by acidifying blood and preventing gill CO2 offloading. A drop checker monitors levels via colour change.
❓ Why It Matters
Planted tanks: primary limiting nutrient. Fish health: acute toxicity above 40 ppm. pH: CO2 dissolves as carbonic acid, lowering pH.
🧪 Testing Methods
Drop checker with 4dKH reference solution + bromothymol blue indicator (blue = low, green = ideal, yellow = high). pH swing method (compare tank pH to degassed water pH).
⚠️ Symptoms of Imbalance
Fish gasping at surface. Green algae over-growth with insufficient CO2. Ph swings.