Join Date: Nov 2007
Location: NYC & Long Island, NY
What is the substrate? Also, its difficult to suggest you using the local white rocks from the beach, without a photo of them. In general, white rocks end up looking kind of scummy as they get covered with algae/diatoms/etc.
I think your hardscape needs vertical height. Either taller rocks or driftwood or something. You can still go with the monoplant look you were hoping for, if you try a different ground cover. You dont provide details on lighting, substrate, or whether you plan on dosing CO2/ferts, so it's hard to say, but there are a lot of groundcovers available, most of which are discussed at length on this site.
There's no reason you can't go with your original idea, with some alterations.