I had sea glass in mine that i gathered from Dominica (very neat black sand beach) it was very smooth, but I ended up taking it out. I had put them in the tank along with other items from the beach. I ended up taking them out, no proof there was a problem, but I had a lot of hair algae that I couldn't pin point the source. No glass in the tank now, and no hair algae either. *shrugs* could be coincedence, but I won't put any in again.