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Anything really, to be honest it's all up to you. If it were me,I'd find something in the middle. Oolite sand (very very fine) can be problems as can crushed coral (large grain size). Find what you like, and stick with it.
So I ended up getting one 30lb bag of the fine sugar gain sized sand for the bottom layer, and 3 @ 40lb bags of the "Seaflor Special" to put on top. This should give me a little extra for my refugium.