the 305 fluval, i guess the 304 is the older model, is 260GPH.
and the 1600 L/H power head is 400GPH.
660 for a 72 is less then 10X turnover very bad for accumulation in sand bed.
did you run your fluval with or without media?
and there wasn't any other circulation? 20X+ is best for reef tanks.
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