PAR = photosynthetically available radiation
PAR is basically how much energy the bulb provides for photosynthesis. Typically the bluer the bulb (ie higher kelvin) the less PAR that is available and can lead to less coral growth and different coloration. For example, PAR-wise, a 175W 10000k, a 250W 14000k, and a 400W 20000k could all have the same PAR values.
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