Yea, it won't ever break down since it's inert. I do find that I need to add some flourite back every once in a while though. Everytime I take out moss tiles to trim I always lose a few pieces that the moss clings too.
Hmm, what about all this ion exchange stuff, though? I never took chemistry, so I'm clueless. I know I used this chart long ago to decide which substrate to go with, and it talked about calcium and iron, blah, blah. This stuff can't be just like plain gravel with all the cost of it, can it?
 Not ions but "Cation Exchange Capacity (CEC.)"