Planted Aquarium Lighting & CO2 Guide: PAR, Bubble Rates & pH/KH Chart
Core Principles of Aquatic Photosynthesis
- Watts Per Gallon is Obsolete: LED efficiency renders watts useless. Measure lighting by PAR (Photosynthetically Active Radiation, 400–700 nm wavelengths) measured at the substrate level.
- The Golden Target: Ideal dissolved CO2 for planted aquariums is 20 to 30 ppm (mg/L). Levels above 35–40 ppm induce respiratory distress (gasping) in sensitive livestock.
- The pH Drop Method: Dissolving CO2 forms carbonic acid, depressing pH. Injecting exactly enough CO2 to achieve a 1.0 pH drop from fully degassed tank water yields ~30 ppm dissolved CO2 regardless of starting water hardness.
- The Algae Triangle: Algae is not caused by high light or high fertilizer; it is triggered by an imbalance between high light intensity and fluctuating, insufficient carbon dioxide.
1. Understanding PAR (Photosynthetically Active Radiation)
Aquatic plants do not care how bright a light fixture looks to the human eye (lumens or lux). Human vision peaks in green wavelengths (~555 nm), which plants mostly reflect. Plants utilize photons in the 400 nm to 700 nm spectrum (blue and red peaks) to drive the light reactions of photosynthesis.
PAR is measured in micromoles of photons per square meter per second (μmol/m²/s). When selecting planted LED lighting, target substrate PAR values:
| Lighting Category | PAR at Substrate Level | CO2 Injection Required? | Suitable Flora Species |
|---|---|---|---|
| Low Light | 15 – 30 μmol/m²/s | No (Low-Tech System) | Anubias, Java Fern, Cryptocoryne, Bucephalandra, Java Moss. |
| Medium Light | 30 – 50 μmol/m²/s | Recommended for compact growth | Amazon Swords, Vallisneria, Ludwigia repens, Dwarf Sagittaria. |
| High Light | 50 – 100+ μmol/m²/s | MANDATORY (High-Tech System) | Rotala H'Ra, Monte Carlo carpets, Cuba (HC), Glossostigma, Alternanthera reineckii. |
2. The Chemistry of CO2 Injection: The pH-KH-CO2 Relationship
When carbon dioxide gas (CO₂) dissolves into aquarium water, a fraction chemically hydrates to form carbonic acid (H₂CO₃), which partially dissociates into hydrogen ions (H⁺) and bicarbonate (HCO₃⁻):
CO₂ + H₂O ⇔ H₂CO₃ ⇔ H⁺ + HCO₃⁻Reversible equilibrium governed by water temperature and carbonate alkalinity buffer (KH).Because carbonate hardness (KH) measures the concentration of bicarbonate ions in solution, measuring your water's KH and current pH allows mathematical calculation of dissolved CO2:
CO₂ (ppm) = 3.0 · KH · 10(7.00 − pH)Where KH is in German carbonate degrees (dKH).3. The pH-KH-CO2 Reference Chart
Use this reference table to evaluate dissolved carbon dioxide based on your water test results:
| dKH \ pH | pH 6.4 | pH 6.6 | pH 6.8 | pH 7.0 | pH 7.2 | pH 7.4 |
|---|---|---|---|---|---|---|
| 2 dKH | 24 ppm | 15 ppm | 9 ppm | 6 ppm | 4 ppm | 2 ppm |
| 4 dKH | 48 ppm | 30 ppm | 19 ppm | 12 ppm | 8 ppm | 5 ppm |
| 6 dKH | 72 ppm | 45 ppm | 29 ppm | 18 ppm | 11 ppm | 7 ppm |
| 8 dKH | 95 ppm | 60 ppm | 38 ppm | 24 ppm | 15 ppm | 10 ppm |
| 10 dKH | 120 ppm | 75 ppm | 47 ppm | 30 ppm | 19 ppm | 12 ppm |
Color Legend: Green (20–35 ppm: Optimal for plant growth); Yellow (35–50 ppm: Monitor livestock breathing); Red (>50 ppm: Dangerous/lethal to fish).
4. The 1.0 pH Drop Rule: Bypassing Organic Acid Distortions
In tanks using active aqua soil (ADA Amazonia, Tropica Soil) or driftwood, humic and tannic acids release organic anions that buffer water without contributing to true carbonate alkalinity. This distorts KH test kits, causing the standard chart to overestimate dissolved CO2.
To verify true 30 ppm dissolved CO2 regardless of substrate type, use the 1.0 pH Drop Method:
- Draw a sample of tank water and let it sit uncovered for 24 hours (or aerate with an air stone for 30 minutes) to off-gas all dissolved CO2. Test the pH (e.g. 7.5).
- Turn on your CO2 solenoid 2 hours before your lights turn on.
- Tune your needle valve until the aquarium water's active pH drops by exactly 1.0 unit below the degassed baseline (e.g., from 7.5 down to 6.5).
- A full 1.0 pH drop represents a 10-fold increase in carbonic acid, guaranteeing approximately 30 to 35 ppm dissolved CO2 at lights-on.
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