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Ecosystem cycles

The CO₂ cycle

In most planted tanks above moderate light, CO₂ is the variable that sets the growth ceiling. This article explains how CO₂ dissolves, how it interacts with KH to shape pH, and how to inject it safely.

The CO₂ cycle: the limiting nutrient in planted tanks

Aquarium CO₂ cycle — dissolved CO₂ shifts between gaseous, dissolved, and biological pools, consumed by photosynthesis and produced by respiration
Aquarium CO₂ cycle — dissolved CO₂ shifts between gaseous, dissolved, and biological pools, consumed by photosynthesis and produced by respiration

There is a phrase that circulates among planted-tank veterans: "Light is the accelerator; CO₂ is the chassis." Tropica's public growth studies through the 2010s confirmed the intuition: above moderate light intensity, CO₂ is the variable that actually limits growth. Adding fertilizer fails. Swapping LEDs fails. Adding CO₂ — and the response is immediate. This guide unpacks what this often-mythologized gas actually does in the tank.

1. CO₂ does not simply "dissolve"

When CO₂ enters water, it reacts. The result is the carbonate-bicarbonate equilibrium system:

CO₂(aq) + H₂O ⇌ H₂CO₃ ⇌ HCO₃⁻ + H⁺ ⇌ CO₃²⁻ + 2H⁺

This chain is one of the oldest chemical equilibria on Earth — it governs ocean pH, lake acidity, and the diurnal pH swing in your tank. In the aquarium-typical pH range of 6.5–8.0, most inorganic carbon exists as bicarbonate (HCO₃⁻), with smaller fractions as dissolved CO₂ and carbonate. But only free CO₂(aq) is efficiently taken up by most aquatic plants (Maberly & Madsen, Aquatic Botany, 1990).

This is why "liquid carbon supplements" sold as bicarbonate sources have limited effect — they provide HCO₃⁻, which most tropical aquarium plants cannot use efficiently. A few hard-water-adapted species (Egeria, Vallisneria) have evolved bicarbonate utilization capacity, but the majority of cultivated stems (Rotala, Eriocaulon, Bucephalandra) recognize only CO₂.

2. KH × pH determines CO₂ concentration

The carbonate equilibrium yields a practical and widely-used estimation for aquarium use. At 25 °C, with KH measured in °dKH and a closed CO₂-injected tank where carbonate chemistry dominates pH:

[CO₂](mg/L) ≈ 12.84 × KH(°dKH) × 10^(6.37 − pH)

This is the basis for drop checker indicators. A drop checker contains a 4 °dKH reference solution with bromothymol blue indicator; only CO₂ permeates the glass membrane. As tank CO₂ diffuses in, the indicator pH shifts and the color changes:

  • Blue: CO₂ < 10 mg/L (too low)
  • Green: CO₂ ≈ 20–30 mg/L (target zone)
  • Yellow: CO₂ > 40 mg/L (dangerous — fish gasping)

Use the water chemistry calculator to derive CO₂ from any KH/pH combination. Caveat: the formula assumes carbonate chemistry is the only pH-controlling system. If you use ADA Aquasoil or peat — both add organic acids — pH will be lower than the carbonate equilibrium predicts, and the formula will underestimate CO₂.

3. Inputs and outputs: four pathways

       [external input]                  [losses]
   CO₂ cylinder → diffuser  ─┐    ┌─ surface gas exchange
   yeast / liquid carbon ────┤tank│── plant photosynthetic uptake
   fish + bacterial respiration ┘    └─ (respiration is actually a net source)
  • Surface exchange is bidirectional. Atmospheric CO₂ at ~420 ppm corresponds to ~0.5 mg/L equilibrium in water. Tanks with injected CO₂ above this are net exporters to the atmosphere; tanks below are net importers. Strong surface agitation accelerates equilibration, which is why high-tech setups suppress the surface ripple to retain injected CO₂.
  • Photosynthesis is the dominant sink. A densely planted tank at midday can consume 5–10 mg/L per hour. This is why continuous injection is required, not periodic dosing.
  • Biological respiration is a secondary source but far below what high-light plant growth requires.

4. Injection methods evaluated

4.1 Pressurized cylinder + solenoid + diffuser (recommended)

The most stable, lowest long-term cost, and most controllable method. A diffuser forces CO₂ through a ceramic plate generating <0.1 mm microbubbles with high residence time and surface area; dissolution efficiency reaches 70–85% (Tropica internal tests, 2018). An inline reactor (external CO₂ reactor) on a canister return achieves >95%, at the cost of plumbing complexity.

4.2 Disposable canisters (e.g., Fluval 88g)

Suitable for tanks <30 L. A canister lasts 1–2 weeks; long-term cost runs 5–10× the cylinder-based system.

4.3 Liquid carbon (Excel, Easy Carbo)

The active ingredient is glutaraldehyde, which is not actually a carbon source for photosynthesis. Glutaraldehyde acts primarily as a mild biocide (algae suppression) and is then degraded by heterotrophic bacteria into small molecules that incidentally provide trace organic carbon (Eisert, 2008, technical analysis). Effective but expensive and harmful to specific sensitive species (Riccardia, Riccia, mosses, and shrimp at over-dose).

4.4 DIY yeast (not recommended)

Fermentation output is uncontrollable. pH swings widely; new hobbyists typically abandon the method within two cycles.

5. CO₂ toxicity thresholds

CO₂ itself is not directly toxic, but it shifts blood pH. Fish excrete metabolic CO₂ across the gills; when water CO₂ rises, the diffusion gradient flattens, and the fish cannot off-load its own CO₂. Blood pH falls — respiratory acidosis. Symptoms: rapid breathing, gathering near the surface or outlet, lethargy, eventual collapse.

Published thresholds (Hesse et al., Aquaculture Research, 2015, review):

  • Most tropical species: CO₂ < 30 mg/L is long-term safe
  • High-density stocking: < 25 mg/L
  • Oxygen-sensitive species (some tetras, Crossocheilus): < 20 mg/L

A solenoid timer is non-negotiable — without one, CO₂ accumulates overnight while photosynthesis is off, and concentrations cross toxic thresholds by morning. Tropica's protocol: CO₂ on 1–2 hours before lights on, off 1 hour before lights off.

6. Calibration procedure

  1. Measure baseline KH and pH (no CO₂ injection)
  2. Install the drop checker and observe for 48 h to establish color stability
  3. Start CO₂ injection, increasing by ~0.5 bubbles per second per day, until the drop checker reads yellow-green
  4. Observe fish. Any surface-gulping, gathering near the outlet, or accelerated gill movement means immediately reduce injection
  5. After a week of stable behavior, lock in the on/off timing relative to the photoperiod
  6. Use the light + CO₂ calculator to balance PAR against CO₂ supply

7. The truth about pearling

Oxygen bubbles on leaf undersides ("pearling") signal supersaturated O₂ in the water column — photosynthesis is outpacing solubility. It is indirect evidence that CO₂ and light are both sufficient, but it is not the only indicator of plant health. Slow-growing plants (Anubias, Bucephalandra) never pearl heavily even when healthy; fast-growing plants (Rotala) pearl easily but are also the first to signal CO₂ shortage with stalled growth.

References

  • Maberly, S. C., & Madsen, T. V. (1990). Affinity of aquatic macrophytes for CO₂ and HCO₃⁻ in relation to their natural habitats. Aquatic Botany, 38(1), 79–99.
  • Tropica Aquarium Plants A/S (2018). Internal R&D reports on CO₂ diffuser efficiency. Unpublished; summarized in tropica.com blog (2018–2020).
  • Hesse, D. N., et al. (2015). Effects of CO₂ on freshwater fish: a review. Aquaculture Research, 46(9), 2027–2042.
  • Stumm, W., & Morgan, J. J. (1996). Aquatic Chemistry: Chemical Equilibria and Rates in Natural Waters (3rd ed.). Wiley-Interscience.
  • Walstad, D. (2013). Ecology of the Planted Aquarium (3rd ed.). Echinodorus Publishing.
  • Eisert, R. (2008). Glutaraldehyde in planted aquaria — mechanism, dosage and side-effects. Practical Fishkeeping, technical brief.
  • Bowes, G. (1985). Pathways of CO₂ fixation by aquatic organisms. In Inorganic Carbon Uptake by Aquatic Photosynthetic Organisms, American Society of Plant Physiologists.

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