Are clam and snail shells harmful in an aquarium?
I received some marine clam and snail shells as a gift. Are there any reasons why I should not use them in a freshwater aquarium?
there are no objections (except that it may appear optically unnatural, but this is a mere question of taste and does not bother the fish), provided the shells are free from oily or other - in the worst case remaining meat - pollutants. The hardness may rise slightly in acidic water, but the effect is usually so weak that it is negligible, at least if you carry out partial water changes at a more or less normal range.
Dr. Bodo Schnell