Also known as the Wood Apple, or Elephant Apple, the Bell Cup is the shell of a fruit produced by Limonia acidissima. These cups are durbale and strong. As far as botanicals go, they are very inate, not really altering water chemistry a great deal. They serve great use as a sheleter, cave or spawning pod for smaller fishes. 
 
For the Biotope specialist, this item is native to Sri-Lanka, India and Bangladesh.