I've always used Carib Sea Special Reef Grade (2mm) aragonite in my CA reactor. I don't know if oyster shells would dissolve properly, or what besides calcium would be the byproduct. Aragonite is cheap enough and lasts long enough for me that this has never been an issue.
You could try it and analyze the output of the reactor.
>I've never seen any discussion of using crushed oyster shells as media in a calcium reactor. Would this be a good media? Bad
media? Why?<
I'd be inclined to recommend against it. I believe that sometimes mollusks concentrate certain toxic substances from the water around them. I wouldn't be surprised if they put some of it in their shell. I'd use (and do) crushed coral since this is likely to provide the elements you need since what you are trying to grow is coral. FWIW, I've found the cheapest Altantic crushed coral seems to work just fine in my reactors. Usually less than $0.40/lb.
