Kelvin (K) measures the color temperature of light. Aquarium lighting ranges from 6500K (sunlight, freshwater plants) to 25,000K (deep blue, reef SPS). Higher K = bluer, lower K = warmer.
Kelvin temperature drives aquarium light selection. Common ratings: 6500K (daylight, perfect for freshwater plants), 8000K (slightly cool, mixed planted), 10,000K (FOWLR + soft reef), 14,000K (mixed reef + LPS), 20,000K (SPS-dominant), 25,000K (deep blue, max coral fluorescence). Why higher K = bluer: the higher the temp, the more blue + violet wavelengths. Counter-intuitive but standard physics term. Plant tanks: 6500-7500K mimics sunlight. Adjust to 7000K for slight blue boost making greens pop. Reef: blue-heavy is preferred because coral pigment proteins fluoresce brightest under 405-450nm wavelengths. Pure white light makes corals look brown + dull. Programmable LEDs: AI Hydra, EcoTech Radion, Reefi Uno - mix individual color channels to achieve custom Kelvin. Most reefers run "ultra blue" (60% blue + UV, 40% white) which reads ~22,000K. T5HO bulbs by K: ATI Blue Plus (most popular reef supplement), Aquablue Special (10K + actinic), Coral Plus (peak photo), Purple Plus (deep blue). Combination supplementation: hybrid LED+T5 or all-T5 fixtures use 4-8 bulbs of varied K + actinic to fill spectrum gaps.
Reviewed by the Fast Aquatics husbandry team · Updated May 2026
Kelvin (K) measures the color temperature of light. Aquarium lighting ranges from 6500K (sunlight, freshwater plants) to 25,000K (deep blue, reef SPS). Higher K = bluer, lower K = warmer. Kelvin temperature drives aquarium light selection. Common ratings: 6500K (daylight, perfect for freshwater plants), 8000K (slightly cool, mixed planted), 10,000K (FOWLR + soft reef), 14,000K (mixed reef + LPS), 20,000K (SPS-dominant), 25,000K (deep blue, max coral fluorescence).
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