PAR and Spectrum Explained — What Your Corals Actually Need

The Numbers Behind the Light

Even the best LED money can buy won't help if PAR and spectrum aren't understood. Getting lighting wrong either bleaches corals or leaves them in the dark.

What Is PAR?

PAR (Photosynthetically Active Radiation) measures the light in wavelengths that symbiotic algae inside corals can actually use. The unit is micromoles per square metre per second (umol/m2/s). Lumens measure brightness to human eyes; PAR measures useful light for corals.

PAR Requirements by Coral Type

• Soft corals and mushrooms: 50–150 PAR — the low-light crowd. Too much light and they shrink and lose colour.
• LPS corals: 100–250 PAR — hammers, torches, frogspawn, acans, brains. Mid-tank placement usually works well.
• SPS corals: 200–450+ PAR — shallow reef crest species that need serious light. Top third of the tank.
• Clams: 200–400 PAR — strong light needed for the symbiotic algae in their mantles.

Measuring PAR

• Apogee MQ-510 — the gold standard (£400–500). Some reef clubs loan them out.
• Seneye — good enough for hobby purposes (£80–150).
• PAR meter rental — several UK reef shops rent Apogee meters for £10–20/week.
• Community PAR maps — search for a specific light model to find published data.

Key Spectrum Wavelengths

• Royal blue (440–460nm): The backbone of any reef light — most efficient for coral photosynthesis.
• Violet/UV (380–430nm): Drives coral fluorescence and contributes to photosynthesis.
• Cyan/green (490–520nm): Penetrates deeper, adds natural appearance.
• Cool white: Adds brightness and makes the tank look more natural to human eyes.

Practical Tips

1. Start with the manufacturer's recommended settings
2. Rent a PAR meter or use a Seneye to measure actual readings at different tank positions
3. Adjust intensity to match coral type requirements
4. Run mostly blue — a 70/30 or 80/20 blue-to-white ratio provides the spectrum corals use most
5. Add UV/violet channels if available — even 20–30% makes a visible difference

More light is not always better. Corals acclimate to specific light levels. Cranking lights to maximum causes bleaching, not faster growth. Ramp up gradually and let coral response guide adjustments.

Browse reef lights from UK sellers — knowing PAR requirements helps with buying the right light the first time.

Got a PAR map worth sharing? Real-world data from UK setups is always more useful than manufacturer specs.