Marine Substrate in a freshwater tank?

[Gix]

I was looking for some crushed coral substrate to harden the water in my crayfish's tank. I can get some regular crushed coral intended for cichlid tanks, which I'm sure would work just fine. The LFS closest to me, however, only has "Florida coral substrate," which is marked for marine tanks. Is there a difference?

 

[Teelie]

It's definitely going to spike your pH up into the 8.2+ range which is too high for some fish but crayfish might adapt. I'm not really certain about them. The marine only stuff may have salts and minerals that would be found in the ocean but not in freshwater normally so it might be risky.