My understanding is that once enough goop (I believe that's the correct technical term) had accumulated in the foam and been broken down that you would have a similar situation to your sand bed in terms of nitrate removal. I am no scientist so unfortnately I cannot give you a better explanation. I just didn't want to end up with a bunch of foam full of crap
if it wasn't appropriate. BTW, how deep is your sand bed?
Thanks for the reply,
Scott.