Are you concerned about the constantly increasing GH the marine substrate will give you? Those shells will be adding calcium to the water constantly.
My wife made a few comments about putting another riparium in our living room today! I told her not to do that unless she is serious - I tend to charge ahead when I get even a slightly green light. If I do so, it will be in a tank shaped similar to the one you show - something about 18 inches front to back, maybe 30 inches long, and not very high, plus rimless. I will be following this one very closely for ideas.