03-24-2012, 06:53 PM
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OK, what you have are 18" T8 (one inch diameter) bulbs, two of these in-line over your tank will not be a problem but for the most light sensitive (e.g. nocturnal) fish - it is a matter of light quantity rather than type (spectral output) and using only two of these rather than all four is low light. You could have them on for probably 8-10 hrs without much algae, though this will be something you will have to experiment with, if a lot of algae develops, cut back an hour on the light, etc.