I wanted to have sand over fertile substrate on my new tank project in my bedroom. Noticed alot of fertile substrate were of courser material and the general opinion/experiance is that snad will gradually sink below.
Dont know if this is any use to the Yankee doodles, but overhere and I guess Europe JBL Aquabasis is a possible subrate option. It is soil like in texture and a clay based substrate. Hopefully it will therefore stay under the sand.