I want to build an iwagumi style tank with a "beach" around my planted area. What substrate should I use for the unplanted beach area? I'll use a commercial aquascaping soil for the rocky "island" that will be planted with a carpet like HC, dwarf hair grass, etc. I like the look of pool sand but crushed oyster shell is similar and might give me some added benefit of helping to buffer the water. Thoughts? Experience with using C.O.S. this way?

The oyster shell should raise the hardness of the water, so keep that in mind.

And exhaust your aqua soil much faster than an inert substrate like sand.

Why would the oyster shell exhaust the aqua soil? Wouldn't it be leaching more calcium into the water as it slowly dissolved?

Some of it by the way of carbonates/carbonate hardness, or dKH. Stuff that aquasoil loves to absorb but only has a limited capacity before the buffering is depleted.

However, I am skeptical if aquasoil that no longer buffers is unable to provide beneficial nutrients to plants.