PAR Requirements: SPS vs. LPS vs. Softies

Run the Reef Lighting PAR Calculator and you'll get a target band: SPS 200-400, LPS 100-250, softies 50-150 µmol/m²/s, consistent ranges confirmed across independent sources including ReefCo Aquariums and Aquarium Specialty. This article covers the part the calculator can't: why the gap is that large, and how to move a coral into a new PAR range without bleaching it.

The actual mechanism, not just habitat depth

The usual explanation stops at "SPS lives in shallower, brighter water." That's true but incomplete. A 2019 study on coral photoacclimation published in Royal Society Open Science found that corals acclimated to high light don't necessarily pack more zooxanthellae per unit of tissue. Instead, shallow, high-light colonies grew roughly five times more tissue volume, expanding their total symbiont population through growth rather than density, while each algal cell ran about 60% leaner on chlorophyll since it wasn't scavenging for scarce photons.

Light also directly subsidizes growth. A 2014 study in PeerJ measured lit coral calcification at roughly 23.7 µmol CaCO₃/day/cm², about double the dark rate, because calcification in the dark is oxygen-limited without photosynthesis supplying it. That's the real reason SPS, which builds dense calcium carbonate skeleton fast, needs more light than a soft coral building little to no skeleton at all.

Why sudden PAR jumps cause bleaching

A 2013 study in PLoS One tracked Acropora photosynthetic response to light and found it correlated with cumulative exposure over the prior one to three days, not the current instant. A coral's zooxanthellae population and pigment ratios need time to re-equilibrate. Move a coral from 100 PAR to 300 PAR overnight and its biology hasn't caught up, which is the direct mechanism behind light-shock bleaching, not just a vague "too much light" warning.

Current USA is direct about this: overacclimating too fast is a real, fast-acting way to bleach or kill a coral, not a safe default. Their own acclimation guidance calls for raising the fixture and lowering it gradually over 1-2 weeks, or ramping intensity by roughly an hour's worth of increase per week.

A real acclimation schedule

1. Start low, and start at the bottom. Place new corals on the tank floor first, per LiveAquaria's acclimation guide, which is the lowest-PAR position in most tanks.
2. Cut photoperiod, not just intensity. Reduce photoperiod by about a quarter in week one, adding roughly an hour back per week as the coral settles, per the same guide.
3. Move gradually over weeks, not days. Physically inch the coral toward its final, higher-light position over the following month rather than relocating it in one move.
4. Ramp intensity at the start and end of the day too. An hour-long sunrise/sunset ramp, matching EcoTech's own default Radion schedule (a 1-hour ramp up, 8 hours steady, 1-hour ramp down), reduces daily stress on top of the multi-week ramp.

The bottom line

SPS needs more light because it grows more tissue and more skeleton, and photosynthesis directly powers both. That's biology, not convention. Whatever band you're targeting, get there over weeks, not days, since your coral's light response runs on a delay you can't see happening in real time.