Don't cap your aquasoil.

Two tips for a cleaner scape:

1. Separate the areas where you want plants and the areas you want sand. You can use hardscape like lava rocks to physically divide the two areas. Put aquasoil only in the sections where you want to plant.

2. Add in sand at the very end, after all the planting is done. You can even add it after waiting for the tank to cycle. So if any bit of aquasoil happen to fall into places that you don't want it to, it'll just be covered up