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Is your tank 24 high, or 18 high?
It it is 18 high, meaning 24" front to back, then each bulb is lighting exactly 24x24" so it wouldn't matter much.
I voted parallel because I don't keep much stuff right up against the glass, more stuff in the middle, so I wanted the bulbs running down the middle of the tank. This may be negated by a reflector, but my PFO reflectors are meant to run parallel so that iw hat I do.
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