Introduction

Photosynthetically Active Radiation (PAR) plays a crucial role in reef aquariums by providing corals with the light energy needed for photosynthesis. Understanding and managing PAR levels can mean the difference between a thriving reef and one struggling with poor growth and bleaching. This guide explores how PAR works, how to measure it, and how to optimize it for different coral types.

Additionally, corals are not entirely dependent on light alone. They can also supplement their energy needs by consuming plankton and organic nutrients. This article integrates research-backed insights on PAR, coral feeding strategies, seasonal variations in light, and stress responses, ensuring hobbyists have a comprehensive understanding of how to optimize coral health.

What Is PAR and Why Does It Matter?

PAR refers to the portion of the light spectrum (400–700 nanometers) used by photosynthetic organisms, including the symbiotic algae (zooxanthellae) living within coral tissues. These algae convert light into energy, sustaining both themselves and their coral hosts.

Without sufficient PAR, corals can experience:

✅ Stunted Growth – Lack of energy limits tissue expansion and skeletal growth.
✅ Bleaching – Corals expel their symbiotic algae when light conditions are too extreme.
✅ Color Loss – Insufficient PAR can lead to dull, brownish hues instead of vibrant coloration.

PAR vs. PUR vs. Lux: What’s the Difference?

While PAR is a useful measure, it's important to distinguish it from other lighting terms:

Key Takeaway: High PAR doesn’t always mean better results. Coral growth depends on the right wavelengths of light, particularly blue (450–470 nm) and red (620–660 nm), which optimize photosynthesis.

How to Measure PAR in Your Reef Tank

Measuring PAR is crucial for ensuring your corals receive optimal lighting.

Using a PAR Meter

A PAR meter allows hobbyists to measure light intensity at different points in the tank. Follow these steps:

1. Calibrate the Meter – Ensure accuracy before use.
2. Take Readings at Multiple Depths – Light levels decrease with depth due to water absorption.
3. Check for Shading and Hotspots – Rockwork and coral placement can cause uneven lighting.
4. Adjust Lighting Accordingly – Raise, lower, or modify settings based on readings.

Ideal PAR Ranges for Corals

Different corals have varying PAR needs:

✅ Placement Tip: SPS corals thrive under intense light near the top, while soft corals and LPS corals do better in shaded or lower-light areas.

Factors That Affect PAR in a Reef Tank

1. Water Clarity

• Suspended particles, tannins, and organic debris scatter or absorb light.
• Solution: Use Reef Clarify and regular water changes to improve clarity.

2. Tank Depth and Aquascaping

• Light intensity decreases with depth.
• Large rock formations create shaded areas.
• Solution: Use supplemental lights or reposition corals based on PAR needs.

3. Type of Lighting

4. Aging Bulbs and LEDs

• LED efficiency decreases over time.
• T5 and metal halide bulbs degrade, requiring replacement.
• Solution: Test PAR annually and replace bulbs as needed.

Coral Feeding: How Live Feeds Enhance Growth and Color

While PAR is crucial, corals are also heterotrophic, meaning they capture and consume organic particles to supplement their diet.

1. The Role of Phytoplankton in Coral Nutrition

✅ Boosts Zooxanthellae Health: Provides essential nutrients for photosynthesis.
✅ Enhances Coral Coloration: Carotenoids and pigments in phytoplankton enhance red and yellow hues.
✅ Supports Larval Development: Essential for coral spawning events.

2. The Importance of Rotifers for Coral Growth

Rotifers are small zooplankton rich in proteins and lipids, ideal for SPS and LPS corals.

✅ Increases Coral Growth Rate – Helps build skeletal mass in SPS corals.
✅ Supports Tissue Regeneration – Provides amino acids for healing.
✅ Stimulates Polyp Extension – Encourages active feeding behavior.

Coral Stress Responses to PAR and Nutritional Changes

When corals experience sudden changes in PAR or feeding, they exhibit:

• Bleaching: Corals expel zooxanthellae under extreme stress.
• Photoinhibition: Overexposure to high PAR damages photosynthetic proteins.
• Pigment Loss or Color Shifts: Insufficient nutrients cause duller coral coloration.

✅ Solution: Adjust PAR gradually and provide balanced feeding to prevent stress.

How to Create a Custom PAR and Feeding Regimen

✅ Use a PAR meter to measure intensity at different depths in your tank.
✅ Implement target feeding schedules based on coral type.
✅ Increase PAR levels gradually to avoid photoinhibition.

Example PAR & Feeding Schedule

Common PAR-Related Issues and Solutions

Conclusion

Mastering PAR and feeding techniques is essential for a successful reef aquarium. By understanding its impact, measuring it correctly, and optimizing light settings, hobbyists can create a thriving coral ecosystem.

By combining strategic lighting adjustments with a varied diet, reef hobbyists can maximize coral health and coloration, ensuring a vibrant and healthy reef tank.