Gravel under AS would be fine, or small lava rocks or pumice. ADA's power sand that's made to go under it is basically pumice rocks or something like that. Gravel wouldnt have the cec capacity of lava rocks or pumice but structurally speaking it should work just fine.
If you cap Aquasoil off with something heavy such as gravel, it'll cause the AS to break down a lot faster and turn to mush. Plus the gravel being heavier it's going to work its way to the bottom and the lighter aquasoil will go to the top. The first time you uproot something it'll be all mixed together. So I wouldnt do that.
Thanks I think I'll do that.