I was under the impression that grit and oyster shell are two different things...Right? My DH picked up some grit from the feed store today but it looks exactly like the oyster shell. Now the oyster shell came in a sealed bag with oyster shell written all over it but the grit was in a genric bag, scooped out of a separate container. Are they supposed to look the same??? I have been giving them sand up until now but thought I had better switch them to something larger. Your advice is always appreciated.