That's a deeper tank, I'd go with 3 x 400w. color temp is simply your personal preference. 10k are whiter and brighter than 20k, but white to the point of almost yellow, depending on bulb. Radiums are really nice blue, but less intense and need replacing every 9 months (ideally).
A nice compromise might be a 14k bulb.
Keep in mind reflectors can make or break your bulb choice.
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Brad
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