Daily light integral from a PAR reading and a photoperiod, against what different corals want.
| Coral group | PAR | DLI | Notes |
|---|---|---|---|
| Mushrooms, ricordea | 50 to 100 | 2 to 5 | Bleach easily, shade them |
| Leathers, zoanthids | 75 to 150 | 3 to 7 | Tolerant of a wide range |
| LPS, euphyllia | 100 to 200 | 4 to 9 | Lower third of the tank |
| Mixed reef average | 150 to 300 | 6 to 13 | Middle of the rock |
| SPS, acropora | 250 to 450 | 10 to 20 | Top third, needs stability |
| Tridacna clams | 300 to 500 | 12 to 22 | Highest demand in the tank |
Inside the range. Hold it there. Corals adapt to consistency far better than to a better number reached quickly.
DLI = 250 µmol/m²/s × 9.0 h × 3600 s ÷ 1,000,000 = 8.10 mol/m²/day
PAR is an instantaneous rate, photosynthetically active radiation in micromoles of photons per square metre per second. DLI is the total those photons add up to over a day, in moles per square metre. DLI = PAR × seconds ÷ 1,000,000.
DLI is the more useful number because it captures the trade a photoperiod makes. Running 200 PAR for ten hours and 285 PAR for seven hours deliver the same daily total, and corals largely respond to the total.
A ramp does not deliver full intensity. Counting ramp hours at half their clock time is a reasonable approximation for a linear ramp, which is what most controllers run.
PAR falls off fast with depth and with distance from centre. A fixture reading 400 at the surface can read 120 on the sand and 60 in the corners. Measure at the coral, and measure in several places rather than one.
The ranges here are collected from what reefers report and from fixture manufacturer guidance. They are wide because acclimation history matters more than the number. A coral grown at 150 PAR will bleach at 300, and the same species grown at 300 will be fine.
When adding a new fixture, start at roughly half the intensity you are aiming for and raise it ten percent every week or two. Bleaching from a fast increase takes months to recover from and often does not.
PAR meters disagree with each other, particularly across LED spectra. Apogee and Seneye read differently on the same fixture. Use one meter consistently and treat the absolute value as approximate.