+1 to the sand issue ^
I use crushed oyster shell (sold at feed stores, used as a supplement to chicken food) in my cichlid tanks to keep the pH high without having to buffer. Here are some of the basic issues I would think your going to face with this.
1. oyster shell is composed of calcium carbonate, and even the crushed stuff is fairly large ~ therefore, its going to work in you tank very similar to CC rock. If not properly maintained with regular vaccuming, it will accumulate debris and become a nitrate trap.
2. a lot of this stuff is light enough it will actually float around your tank (I had to bury it under gravel.
3. Because it is calcium carbonate, it is going to break down overtime (which does add calcium to your tank and will help you with keeping elevated calcium levels), but its going to break down faster than the CC gravel (cause its smaller pieces) and you will have to replenish it.
4. Also, because it is calcium carbonate, I would think that in a SW tank, it would cover fairly quickly with coralline (just as snail shells will), and the whole bottom of your tank will be purple ~ might be a cool look, but coralline is also a cemeting agent, which means it would be cementing the shell together, thus preventing sand dwellers (like microstars and nassarius snails) and water flow movement through the substrate.
Just a few of my thoughts.
If your looking to do it cheaper than buying live sand, go buy a bag of the dry aragonite sand and seed it with a small quantify of true live sand, it will become live.
