The conclusion I have reached is that the sand layer should be quite deep. I am using Tropica Complete Substrate under the sand, then a thick layer of Tropica Soil and ADA Aquasoil. I want the nutrient rich silty layer right underneath the sand for when the Monte Carlo roots down. Sand is entirely permeable and will allow nutrients from the soil to enter the water column. The Tropica substrate has finer particles that benefit root surface area and because they are clay, will slow the rate of nutrients migrating up from the aquatic soils underneath. Simples.