Aquarium lighting serves three roles: visibility, photoperiod (day/night cycle for fish health), and photosynthesis (plants + corals). Fish-only freshwater: any 5-15W LED gives the fish a normal day/night cycle. 6-8 hour photoperiod. Avoid leaving lights on 24/7 - it stresses fish + grows algae. Low-light planted: 20-40 PAR at substrate. Anubias, java fern, crypts, vallisneria thrive. ~25W LED on a 20-gallon long. 7-9 hour photoperiod. High-light planted: 60-120 PAR at substrate. Carpets, reds, demanding stems. Twinstar, Chihiros WRGB II, Aquasky 2.0 G7, ONF Flat One+. CO2 mandatory at this level. 8-10 hour photoperiod. Reef tank: 200-450 PAR depending on coral type. Softies need 50-150 PAR, LPS 150-250, SPS 250-450. AI Hydra, EcoTech Radion, Reefi Uno, Kessil A360. 8-10 hour photoperiod with sunrise/sunset ramp. Photoperiod consistency: use a programmable timer or smart plug. Inconsistent lighting causes algae blooms and stresses fish. Sunrise + sunset ramps reduce stress and look natural.
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