When you use the term "sand", are you referring to the very fine playbox-type material or something coarser? If the former, wouldn't an inch of it over the soil-sand substrate restrict the flow of nutrients from the water column to the substrate?

I use 1.0 to 1.5 inches of 2-3 mm gravel over soaked topsoil, and I have never had a problem with the soil getting into the water column. Of course, I don't do a lot of uprooting of established plants. Some, though.

If a small amounts of soil should get into the water column, it is readily removed by the filter.

Bill