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Depending on how hard your source water is, it may not take that much to boost the pH up to 8-8.25. I think having like 50/50 (sand/crushed coral) substrate will be enough, then the sea shells and coral stones will be more of an aesthetic issue.

Also your filtration is more than good. Just staty clear of the osmosis stuff and as Shawine says, don't worry too much. After all, high pH is far easier and much less complicated than low pH.