Refugium Lighting for Macroalgae: 5000K-8000K, Chaeto Growth Spectrum

Refugium Lighting for Macroalgae: 5000K-8000K, Chaeto Growth Spectrum | Aqualista

🌿 Grow chaeto and macroalgae fast — Learn the ideal spectrum (5000‑8000K), PAR requirements, and photoperiod for a productive refugium that exports nutrients and reduces algae in your main tank.

What Is a Refugium and Why Does It Need Special Lighting?

A refugium is a separate zone (often sump) where macroalgae like chaetomorpha grow, absorbing nitrates and phosphates. It requires moderate light (50‑150 PAR) in the 5000‑8000K range.

Refugiums export nutrients, provide pod habitat, and can run on a reverse photoperiod to stabilize pH. Unlike display reef lights that favor blue for corals, macroalgae need warm white / daylight spectrum (5000‑8000K). Reef2Reef refugium guide. Chaetomorpha and Caulerpa species have chlorophyll peaks similar to terrestrial plants – they absorb red and blue but perform best under full spectrum with a slight warm bias. Using a high‑K (blue) reef light over a refugium will stunt growth and may cause die‑off.

What Is the Best Light Spectrum for Chaetomorpha Macroalgae?

Chaeto grows fastest under 5000‑6500K (daylight) with strong red and blue peaks. Avoid 10000K+ blue‑heavy lights – they suppress growth and cause chaeto to turn pale.

Chaetomorpha contains chlorophyll A and B, which absorb light at 430‑460nm (blue) and 640‑660nm (red). A 6500K LED provides a balanced ratio. BRS tests show that a 5000K warm white lamp grows chaeto 40% faster than a 10000K lamp, and 70% faster than a blue actinic light. Many refugium specific LEDs (e.g., Kessil H80, Tunze Eco Chic) use 5500‑7000K diodes. You can also use a simple 5000‑6500K shop light or a plant grow bulb. Avoid “red/blue only” blurple lights – they work but look unnatural and may not cover the whole spectrum needed for nutrient export.

🌿 Optimal spectrum for macroalgae – 5000‑6500K (daylight/warm white)
– Strong red (660nm) and blue (450nm)
– Avoid actinic blue (420‑470nm only)

What PAR Does Chaeto Need for Maximum Growth?

Chaetomorpha requires 50‑150 PAR. At 50 PAR, growth is moderate. At 100‑150 PAR, growth doubles. Above 200 PAR may cause photo‑inhibition and yellowing.

Unlike corals, chaeto can tolerate higher light but still has an optimum. 2Hr Aquarist study shows that increasing PAR from 50 to 120 doubles the weekly biomass harvested. However, intense light (200+ PAR) can cause chaeto to bleach and fragment. Place a simple PAR meter at the water surface of your refugium. For a typical sump, a 15‑25W LED floodlight at 6‑8 inches distance provides 80‑120 PAR. If your chaeto is turning pale green or white, reduce light intensity or raise the fixture. If it's dark green and not growing, increase light or replace old bulbs.

PAR range	Chaeto growth rate	Appearance
30‑50	Slow	Dark green, dense
50‑100	Moderate	Bright green, healthy
100‑150	Fast	Lime green, vigorous
>200	Stunted / bleaching	Pale, white tips

What Photoperiod Should You Run for a Refugium?

Run refugium lights 12‑16 hours per day. Many reefers use a reverse photoperiod (lights on when display lights are off) to stabilize pH and reduce CO₂ buildup.

Chaeto grows best with longer photoperiods. 14 hours is common. Running the refugium opposite the main display (e.g., display 8am‑8pm, refugium 8pm‑8am) prevents pH drops at night because macroalgae produce oxygen when lit. Melev’s Reef recommends a minimum 12 hours. If you notice chaeto growth slowing, increase photoperiod to 16 hours. Use a simple timer. Do not run 24/7 – macroalgae needs a dark respiration period. For nutrient export, consistency matters more than exact hours. Start at 12h, observe growth, adjust to 14h if needed.

🌙 Reverse photoperiod benefits: Maintains higher pH (0.1‑0.2 increase), reduces algae in display, and provides constant pod availability.

What Are the Best Refugium Light Fixtures?

Top choices: Kessil H80 (adjustable spectrum), Tunze Eco Chic (LED strips), AI Prime Fuge, or simple PAR38 5000K floodlight. For budget, use a 5000‑6500K LED corn bulb.

Kessil H80 Tuna Flora provides 6500‑8000K and strong red peaks, specifically designed for macroalgae. Tunze Eco Chic 8850 offers flexible strips. For DIY, a PAR38 15W 5000K bulb in a clamp light works well. Ensure the fixture is splash‑proof (IP65 or higher) if placed near water. Many reef sumps use inexpensive LED floodlights from hardware stores – look for 5000‑6500K, 20‑30W. Replace bulbs every 1‑2 years because LED degradation reduces PAR. Avoid blue‑only “actinic” lights; they are for coral fluorescence, not algae growth.

💡 Kessil H80Full spectrum, 6500‑8000K, dimmable, $140

💡 Tunze Eco ChicStrip LED, 5500K, low profile, $80

💡 PAR38 5000KBudget, 15‑25W, screw‑in, $20‑30

Why Is Chaeto Turning Pale or Dying?

Pale or melting chaeto indicates too much light, iron deficiency, or low nutrients. Reduce PAR to 80‑100, dose iron (0.05‑0.1 ppm), or increase feeding/nitrate levels.

Chaeto turns from dark green to yellow or white when stressed. Too much light – move light higher or reduce photoperiod. Iron deficiency – macroalgae needs trace iron; dose a refugium‑specific supplement (e.g., ChaetoGro). Low nutrients – if your display has ultra‑low nitrate (0 ppm) and phosphate (0 ppm), chaeto starves. Increase feeding or dose nitrates. Reef2Reef solutions. Also check flow – chaeto needs tumbling motion to avoid dead spots. If chaeto is dark green but not growing, you may need more light (increase PAR or photoperiod).

🧪 Quick fix: Add a few drops of iron supplement weekly. Target nitrate 2‑5 ppm, phosphate 0.03‑0.1 ppm for healthy chaeto growth.

Can You Use Regular Plant Grow Lights for Chaeto?

Yes. Many plant grow lights (purple/blurple) work, but they are less efficient than 5000‑6500K white LEDs for chaeto. White LEDs also provide better visual inspection.

Red/blue only plant lights have high PUR for chlorophyll but often lack green/yellow, making it hard to see pests or chaeto health. They also may overheat. A 5000‑6500K white LED is equally effective and more pleasant. BRS PAR tests show that a standard 6500K floodlight outperforms many cheap “blurple” grow lights. If you already have a plant light, use it, but set it to “white” or “full spectrum” mode. Avoid lights with only 450nm and 660nm – they will grow chaeto but may cause it to look brownish.

🌱 Acceptable plant lights – 5000‑6500K white LED
– Red/blue (blurple) at moderate intensity
– T5 fluorescent (6500K)

How to Set Up an Efficient Refugium Step by Step

Choose a sump section with moderate flow, add a 5000‑6500K LED at 6‑10″ above water, tumble chaeto loosely, run reverse photoperiod 12‑16h, and harvest 1‑2 cups weekly.

Step 1: Install a refugium section in your sump (or hang‑on refugium). Step 2: Add a light source – PAR38 bulb or strip light, positioned to illuminate the whole area. Step 3: Place a ball of chaetomorpha (do not pack tightly; allow water flow). Step 4: Set timer to run opposite display lights (e.g., 8pm‑10am). Step 5: After 2 weeks, start harvesting 25‑50% of chaeto weekly. Step 6: Monitor nitrate/phosphate – they should drop slowly. Reefkeeping magazine guide. If nutrients remain high, increase light intensity or photoperiod. If chaeto stops growing, add iron or increase feeding. A productive refugium can reduce water changes by 30‑50%.

🌿 Harvesting tip: Remove chaeto by hand or with a fork. Squeeze out water over a bucket. Compost or discard. Regular harvesting exports phosphate and nitrate permanently.

📚 Explore more refugium & reef guides: ← Main Lighting Guide (T1) Reef Hub (T2) You are here Coral PAR requirements Coral acclimation Bleaching prevention Brand hub

📖 References & further reading
• BRS – Refugium lighting guide
• Reef2Reef – Refugium lighting
• 2Hr Aquarist – Chaeto PAR requirements
• Melev's Reef – Reverse photoperiod
• Reefkeeping – Refugium setup
• Kessil H80 Tuna Flora