I ran into one big problem with the foam. It floats. I had to tie down the overflow covers I made to keep them from floating away. So be sure you use more rock than foam so it wont try to leave. Also make sure you get foam that is closed cell or it will break down over a few years.
After my crash a few years ago scrapped it all and redid everything with agrocrete (portland cement, sand and shells) I like it a lot better, but it can be a bit brittle so try to use eggcrate inside to reinforce larger pieces.