Ideal lighting for a reef tank is crucial for the health and growth of corals and other photosynthetic organisms. It involves a balance of light intensity (PAR), spectrum, and photoperiod.
Here's a breakdown of ideal lighting parameters:
1. PAR (Photosynthetically Active Radiation):
PAR measures the amount of light within the 400-700 nm wavelength range that is usable for photosynthesis. Different coral types have different PAR requirements:
* Soft Corals: 50-150 PAR (e.g., Zoanthids, Mushroom corals, Leather corals, Xenia). These corals often inhabit deeper or shaded areas of the reef.
* LPS (Large Polyp Stony) Corals: 75-250 PAR (e.g., Euphyllia (Hammer, Torch, Frogspawn), Lobophyllia, Trachyphyllia, Favia, Acanthastrea, Blastomussa). These corals thrive under moderate light.
* SPS (Small Polyp Stony) Corals: 200-400 PAR (e.g., Acropora, Montipora, Stylophora, Pocillopora). These corals are found in shallow, high-light areas of the reef and require intense light to thrive. Some SPS corals can even tolerate PAR levels up to 700 μmol/m²/s, though careful acclimation is key.
* Clams and Anemones: 150-350+ PAR.
Important Considerations for PAR:
* Depth: PAR drops rapidly with water depth. Measure PAR at the depth where your corals are placed.
* Coverage: Light is most concentrated directly below the fixtures. You may need multiple fixtures to ensure even light distribution across your aquascape.
* Acclimation: Always acclimate corals gradually to new lighting intensities to prevent bleaching or stress. Start with lower PAR and slowly increase it over time.
* PAR Meter: A PAR meter (e.g., Apogee MQ-510) is highly recommended to accurately measure light intensity in your tank and dial in your lighting system.
2. Spectrum:
Corals primarily utilize blue wavelengths for growth and calcification, but a full spectrum is generally recommended for optimal health and coloration.
* Blue Light (450-470 nm): Essential for coral growth and stimulating calcification, particularly for stony corals. High blue spectrum also enhances coral pigmentation and fluorescence, which is why very blue reef aquariums are popular.
* Violet/UV (390-410 nm): Considered very important for coral growth and often kept at 100% intensity for much of the photoperiod.
* White Light (Full Spectrum): While blue is crucial, a balanced full spectrum (including some reds, oranges, and yellows) is beneficial for overall coral health and a natural aesthetic. Many reef tank lights offer presets that aim for an optimal spectrum.
* Color Temperature (Kelvin - K): Reef tank lighting typically ranges from 10,000K to 20,000K, with higher Kelvin ratings indicating a bluer light. 14,000K actinic blue lighting is a popular choice for diverse marine tanks due to its balance of growth and vibrant display.
3. Photoperiod (Light Cycle):
Corals need both light and dark periods for optimal health and growth.
* Total Photoperiod: A total photoperiod of 10-12 hours is generally recommended.
* Ramp Up/Down: Include a 1-2 hour "ramp up" and "ramp down" period at the beginning and end of the photoperiod to simulate natural sunrise and sunset, reducing stress on corals.
* Peak Intensity: The highest intensity of light (main photoperiod) can last for about 6-8 hours within the total photoperiod.
* Dark Period: A period of darkness (approximately 12 hours) is essential for corals to release residual energy generated during photosynthesis.
* Moonlight: If using moonlight, keep it at very low levels and consider it as part of the total daily light energy.
Signs of Incorrect Lighting:
* Too Little Light: Corals may appear pale or brown, exhibit poor growth, or stretch towards the light source (elongation).
* Too Much Light: Corals may bleach (lose color and expel zooxanthellae), retract polyps or soft tissue, or show reduced growth as they divert energy to protective mechanisms.
Ultimately, the "ideal" lighting parameters can vary slightly depending on the specific coral species you keep and your tank's setup. Monitoring your corals and making gradual adjustments based on their health and appearance is key to success.