Lighting is the second-most-expensive equipment line item after the tank itself, and it directly determines what coral you can keep. Under-lit SPS browns out within months. Over-lit zoanthids close up. The right LED for your tank depends on coral type, tank depth, and budget.
The headline number to chase is PAR (Photosynthetically Active Radiation), measured in micromoles. Soft coral: 50-150 PAR. LPS: 100-200 PAR. SPS: 200-400 PAR. Most LED manufacturers publish PAR-at-depth charts - read them carefully.
Soft + LPS only: 100-150 PAR is plenty - one Kessil A360X or AI Prime 16HD covers a 24" cube. SPS-dominant: target 250-350 PAR at frag-rack depth - usually two Radion XR15 G6 or three AI Hydra 32HD per 4-foot tank.
A 75-gallon tank (48"L x 18"W x 21"D) needs different lighting than a 90-gallon (48"L x 18"W x 24"D) even though they're similar volume. The deeper tank needs more powerful fixtures or pendant-mounted spotlights to push PAR to the sand bed.
Plan one fixture per 24" of tank length for SPS reefs. Two Radion XR30 cover 48" with overlap; one Radion plus one Kessil A500X covers 60". For shadow-free LPS reefs, use wider-spread fixtures (Hydra 32HD) over narrower spotlights.
EcoTech Radion XR15/XR30: best all-arounder, app control, 5+ year reliability. AI Hydra 32HD/64HD: powerful, slightly cheaper than Radion. Kessil A360X/A500X: legendary "shimmer" effect, simpler to dial in. AI Prime 16HD: budget pick under $250, ideal for 20-40 gallon tanks. Reef Brite XHO LED bars: supplemental T5-replacement bars for color pop.
PopBloom, Mars-Aqua, ViparSpectra: PAR claims wildly inflated, drivers fail at 18-24 months, no warranty parts. The $200 saved upfront costs you a $400 fixture replacement plus dead coral. Stick with named brands.
Default reef spectrum: 80% blue (450-470nm) + 10% UV (400nm) + 10% white. Use a PAR meter (Apogee MQ-510 $400, or borrow one from a club) to measure actual PAR at frag-rack height. Dial intensity until you hit target PAR for your coral mix.
When upgrading lighting, drop to 50% intensity for week 1, ramp up 10% per week to target. Existing coral bleaches if dropped from 200 PAR (old fixture) to 400 PAR (new fixture) overnight.