shells would be fine for an african cichlid tank they would buffer the pH and increase the hardness but i wouldnt use them in anything but african cichlids
not really but anything with calcium(like seashells) will raise the hardness of the water causing the ph to buffer or tend to stay up. u see hard water tends to be more alcaline(sp?) and soft water tends to be acidic. i dont know if u know this but there actually african cichlids that live in seashells multifactitious(sp?) are really cooll they form little family groups u should think about em im getting some soon. peace