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Happi's advice is good. @ 2x54w - med. light - on a 75 gal. would be just fine, say no more than 8 hrs./day, and you may not need to bother with CO2 - Moderate weekly or semi-weekly dosing with Excel would do the job, IMHO.for 75g go with 3x54w fixture, i think 4x54 will be way too much light to deal with. 2x54w light on 75g should still put you on medium light.