Light and PAR, Explained
There is more mysticism about reef lighting than about anything else in this hobby. It is simpler than it sounds: PAR is how much usable light reaches a spot, and every coral has a range it wants.
What PAR actually is
PAR counts the photons in the 400 to 700 nanometer range that arrive at a point, per second, per square meter. It is not brightness to your eye and it is not watts. A light that looks blinding can deliver less PAR than one that looks dim, because your eye is heavily biased toward green and the coral is not.
Ranges that work
| Group | PAR | Where it goes |
|---|---|---|
| Chalices, Leptoseris, blastos, bounce mushrooms | 40 to 110 | Sand bed, shade under an overhang |
| Acans, scolymia, duncans, ricordea | 50 to 130 | Sand bed or lowest shelf |
| Euphyllia, favia, goniopora | 90 to 170 | Bottom to mid rock |
| Zoas, palys, leathers, GSP | 100 to 200 | Mid rock, wherever there is flow |
| Montipora | 150 to 300 | Mid to upper rock |
| Acropora | 250 to 400 | Top third, near the surface |
| Anemones (bubble tip, carpet) | 150 to 350 | They choose, so light the whole area |
Spectrum is for color, intensity is for growth
Blue heavy light makes fluorescent pigments fire, which is why everything looks better under actinic and why we shoot two photos of every coral. Growth comes mostly from total intensity and stable chemistry. A tank run at 20K blue will grow slower than the same tank at a whiter spectrum, and it will look better doing it. Both choices are valid, just know which one you made.
Acclimating a coral to light
- Start every new piece low, on the sand, whatever its final home will be.
- Move it up a few inches every three or four days.
- Watch the tissue, not the calendar. Pale or washed out color means it is getting more than it wants, so drop it back down for two weeks.
Bleaching is a coral expelling its zooxanthellae because the light exceeded what it could process. Tissue that stays white for more than a couple of weeks usually does not recover.
Photoperiod
Eight to ten hours at full intensity, with a ramp up and down of an hour on each side, is plenty for any of the corals we sell. Longer does not mean more growth, it means more algae. Keep the schedule identical every day: an inconsistent photoperiod stresses coral the same way an unstable alkalinity does.
Do you need a PAR meter
Not to start. Borrow one from a club, or use your light manufacturer's published PAR map with a healthy dose of skepticism, then let the corals correct you. A pale acro at the top and a browning acan at the bottom are more reliable than any spec sheet.