Planted Tank Lighting Explained: Spectrum, PAR & Photoperiod

Lighting drives photosynthesis and coloration. Understanding spectrum (Kelvin), PAR (usable light intensity), and photoperiod timing helps avoid algae while maximizing growth.

Key Concepts

Term Definition Impact
Spectrum (Kelvin) Color temperature Visual tone & plant response
PAR (µmol/m²/s) Photosynthetically Active Radiation Growth rate potential
Photoperiod Hours lights on Energy budget; algae risk
Intensity Distribution Uniformity front/back Even plant development
Plant Type PAR at Substrate
Low Light (Anubias, Fern) 20–40
Medium (Crypts, Most Stems) 40–60
High Demand (Carpeting, Reds) 60–90

Spectrum Selection

5000–7000K full-spectrum LEDs render natural color and support balanced growth. Supplemental RGB or violet diodes enhance reds/purples in demanding stems.

Photoperiod Strategy

  • Start at 6 hours (new tank)
  • Increase to 8 hours after algae control and plant establishment
  • Split photoperiods rarely needed; continuous is simpler

Balancing Light & CO2

Strong light without CO2 injection leads to stalled, pale growth and algae. Match higher PAR with reliable CO2 (see CO2 guide) and balanced fertilizers.

Reducing Algae Risk

Problem Cause Adjustment
Green Dust Bloom Too long photoperiod early Reduce to 6 hours temporarily
Hair Algae High light + unstable CO2 Stabilize CO2 start time
BBA Excess light during low flow zones Improve circulation; moderate intensity

Measuring PAR

Use a PAR meter or reference manufacturer data. Measure at substrate and midwater. Adjust fixture height or dimming to hit target ranges.

Fixture Placement Tips

  • Center LED for even spread
  • Slight forward angle reduces rear shadowing
  • Avoid glass top condensation buildup (reduces intensity)

Final Thoughts

Tune lighting gradually: observe plant response (new growth, color) over weeks. Light is a steering wheel—precise adjustments, not abrupt swings, yield long-term success.