Reef Lighting, Flow and PAR Explained
🐠 The short version: corals run on light and flow, and both are matched to the coral. As a rough ladder, soft corals and LPS want lower-to-moderate light; SPS want more. Whatever the fixture, acclimate light slowly (bleaching = too much, browning = too little), and aim for varied, turbulent flow with no dead spots so corals sway rather than get sandblasted.
🔧 The coral placement, light & flow guide gives the sourced light band, flow band and tank zone for each of our corals — use it to place a coral or fill a spot.
PAR: the light that actually counts
Corals are powered by tiny photosynthetic algae (zooxanthellae) living in their tissue, so what matters isn’t how bright the light looks to you — it’s PAR (Photosynthetically Active Radiation), the light corals can use. Watts and lumens are poor proxies; PAR is the real measure. You don’t need to obsess over exact numbers to start, but understanding the ladder helps you buy the right light and put corals in the right place.
The rough PAR ladder
Light needs climb as you move up the coral difficulty ladder. Treat these as starting bands, not laws — the placement guide carries the sourced figure per coral:
- Soft corals (mushrooms, zoas, leathers) — lower light. Forgiving; great for a young tank.
- LPS (hammer, torch, duncan, acan) — low-to-moderate light and gentler flow.
- SPS (montipora, birdsnest, acropora) — moderate-to-high light and strong, turbulent flow. The demanding end.
Two big caveats: PAR at the coral depends on your fixture, its height, and water clarity, and it varies across the tank (bright up top, dimmer on the sand). That’s why a borrowed PAR meter beats guesswork — but the bands above get you safely started.
Acclimate light slowly
New corals — and any tank getting a new or stronger light — need gradual light acclimation. Too much, too fast, and corals bleach (expel their algae and go pale/white). Too little and they brown (over-produce algae). So start intensity lower and raise it over weeks, and read the coral: pale = back off, brown = give more. SPS especially punish a fast jump in PAR.
Flow: varied, turbulent, no dead spots
Flow does three jobs: it brings corals food and gas exchange, carries away waste, and keeps detritus from settling. The goal is varied, turbulent movement across the whole tank — no dead spots where gunk collects, and no single jet blasting one rock.
- Too little flow → detritus settles, cyano films low spots, corals starve.
- Too much flow → corals stay closed, delicate LPS tear.
Different corals like different flow (softies and LPS gentler, SPS stronger), so aim for good overall movement and then place each coral in the flow it prefers — again, the placement guide tells you which zone suits which coral.
Putting it together
Buy lighting for the corals you’ll actually keep (don’t over-buy on day one), set varied turbulent flow, then place corals by their light and flow needs and acclimate any new light slowly. Get those three right and coral-keeping becomes far more forgiving. Ready to choose corals? See coral care for beginners.
Sources & further reading
- Coralkin — Coral placement, light & flow guide — sourced PAR, flow and placement per coral, drawn from our care sheets.
- Coralkin — Coral care questions — quick answers on light, flow and placement.
- Bulk Reef Supply — PAR, reef lighting and flow guides.
- Randy Holmes-Farley / reef references (via the placement guide and parameter chart citations).
Common questions
What is PAR and why does it matter?
PAR (Photosynthetically Active Radiation) measures the light corals can actually use for photosynthesis. It matters more than watts or brightness to your eye, because corals are powered by light-driven algae in their tissue. Roughly: soft corals and many LPS want lower PAR, SPS want more.
How much PAR do my corals need?
As a rough guide, soft corals and LPS are happy in lower-to-moderate light while SPS want moderate-to-high light. Exact numbers vary by coral and by your fixture, height and water clarity, so treat published bands as a safe starting point and confirm with a borrowed PAR meter.
How do I acclimate corals to new lighting?
Slowly. Start lower and raise intensity over weeks, because too much light too fast bleaches corals. Bleaching (going pale/white) means too much light; browning means too little. Adjust gradually and read the coral.
How much flow does a reef tank need?
Enough for varied, turbulent movement with no dead spots, so detritus stays suspended and corals sway gently rather than being blasted. Flow needs differ by coral, so aim for good overall movement and place each coral in the flow it likes rather than chasing one number.
Can too much flow hurt corals?
Yes. A coral getting sandblasted will not open or extend its polyps, and delicate LPS can tear. If a coral stays closed, try moving it to gentler flow. The right flow lets a coral sway naturally and keeps its surface clean.