Live rock has no place in the sump. I don't know where the idea came from (although I suspect it's a practice developed by stores just to sucker people into buying more rock) but it's ridiculous. Your main tank should be able to hold more than enough live rock to support overstocking, should it appeal to you to do so.
Go with a high quality (important) carbon. Figure out what you want to do with the remaining space in terms of equipment. Don't waste your time with MM in that sump.
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This and that.
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