[piercho]

I've never used clay. I assume you mean laterite, or iron-rich clay used in FW planted tanks. I've used fine silica sand and fine aragonite sand, 3"-6" deep (varied due to tank currents). Seagrass grew in both. Once as an experiment I grew star grass in clay gravel for freshwater tanks. Fed inorganic nutrients, star grass grew in that as well. I did supplement iron for seagrass, using Kents iron supplement and Azoo iron supplement. I went a long time without supplementing iron and it was during that time that the shoal grass dominated the star grass in my aquarium. I suspect that there is a fair amount of iron recycling in the deeper parts of the sandbed, and some grass (shoal grass) are better suited to take advantage of that than others.