Reef lighting is more than “blue looks nice.” Spectrum controls coral growth, color, and algae pressure. This guide breaks down what each band does and how to set it.

What “spectrum” means in a reef tank

Spectrum is the mix of wavelengths your light produces. Reef LEDs blend channels like violet, royal blue, blue, cyan, white, and red. Corals use light for photosynthesis through their zooxanthellae.

Most useful reef energy sits in 400–500 nm. This range drives strong fluorescence and growth. Violet and near-UV (400–430 nm) help many pigments “pop.” Royal blue (440–460 nm) is a workhorse band.

White channels add broad spectrum and improve color rendering. Too much white can raise algae risk in nutrient-rich tanks. Red and green add balance for your eyes. They are easy to overdo.

Separate spectrum from intensity. Intensity is PAR at the coral. Spectrum is the ratio of channels. Stable intensity matters more than chasing perfect color each week.

• Useful spectrum target: heavy 420–470 nm output for most reefs
• Keep red low: 0–5% is often enough for viewing
• Change slowly: adjust one channel by 5% per week

For more on stability basics, review our reef tank water parameters guide. Light changes can stress corals like parameter swings. Treat both with the same caution.

Practical spectrum settings for softies, LPS, and SPS

Start with a proven “blue-heavy” profile. Use violet 80–100%, royal blue 80–100%, and blue 60–90%. Keep white moderate at 10–30% for most tanks. Keep red and green very low.

Match spectrum to PAR and coral type. Soft corals and many LPS do well at 50–150 PAR. Many SPS prefer 200–350 PAR once adapted. Measure PAR if you can. Borrow a meter from a local club.

Use a consistent photoperiod. Aim for 9–10 hours total with a 6–8 hour peak. Add 60–120 minutes of ramp time. Ramps help fish and reduce sudden stress.

Example: a mixed reef at 24 inches deep. Set peak blues at 90% and whites at 20%. Place SPS in the top third at 250–300 PAR. Place LPS mid-level at 100–150 PAR. Place softies lower at 60–100 PAR.

• Softies: 70–90% blues, 10–25% whites, 50–120 PAR
• LPS: 80–100% blues, 10–30% whites, 80–180 PAR
• SPS: 90–100% blues, 5–20% whites, 200–350 PAR

If you are still dialing in nutrients, read our nitrate and phosphate in reef tanks article. High nutrients plus high white light often fuels film algae. Blue-heavy profiles can be more forgiving.

Troubleshooting color, growth, and algae with spectrum

Brown corals often point to excess nutrients or low light. Check nitrate at 2–15 ppm and phosphate at 0.03–0.10 ppm. Then confirm PAR. Many “spectrum” problems are really intensity or nutrient problems.

Bleaching usually means too much light or a fast change. Reduce peak intensity by 10–20%. Shorten peak by one hour. Keep spectrum stable during recovery. Give two to four weeks to judge results.

If algae spikes after a lighting upgrade, look at white and red channels. Drop whites by 5–10% and keep reds near zero. Improve flow and export. Clean the glass and change filter media more often.

Watch for “shadowing” in dense SPS. LEDs can create dark spots under branches. Add a second fixture or raise the light 2–4 inches. You can also add a diffuser. This improves spread and reduces hot spots.

• Acclimation rule: start at 50% intensity for new lights
• Increase pace: add 5% intensity per week for 6–8 weeks
• Placement tweak: move corals 2–3 inches per week, not more

Quarantine and dips do not fix light stress. But they prevent confusion from pests. See our coral dip guide if a coral stays closed. Rule out flatworms and nudibranchs.

Spectrum is a tool, not a magic switch. Keep blues dominant, whites moderate, and red minimal. Measure PAR, change slowly, and track results. Your corals will reward steady hands.

Sources: Dana Riddle, “Lighting the Reef Aquarium” (articles); Sanjay Joshi, reef lighting and PAR measurement presentations; Borneman, “Aquarium Corals” (husbandry references)