10-19-2009, 08:01 PM
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Unless you have some sort of barrier between the layers of gravel and sand, they will mix. And although I've never mixed them, I suspect the end result will be like mixing various sized gravel and pebbles, the small sand will be on the bottom and the gravel mainly on the top.
Unless the gravel is rough or sharp-edged, the Corydoras should not have a problem. If you want sand, I would go with just sand, something like the play sand from Home Depot. It is inert (won't change the water chemistry like marine sands and some pool sands, etc.) and dark coloured so it looks natural.
Sand does have its problems; not insurmountable, but you have to take more care in syphoning so as not to syphon up the sand. In another thread here it was mentioned that sand gets sucked into filters and can damage the impeller. Then there is the compaction of sand which is more likely to occur than with gravel. There was a good thread not long ago about sand and maintenance, you should have a read. Can't remember who posted it, but it was good.