So I did my own little experiment. Put a foam block in a bucket of water weighed down by one large lava rock. The answer is....it depends. The rock may hold it down if positioned correctly. However, if its off by a bit, the foam block shoots back up. The foam blocks in my tank seem to be very securely held down by rock but I'm not sure I want to have a miniature earthquake during flooding.......so I'm ripping up the mid section to remove the blocks. Thanks for the feedback guys.