Leaves in a tank replicate the leaf litter zone of tropical fish habbitats (mainly jungle pools and streams). Many species of dwarf cichlids dwell in this enviroment as well as some catfish and characin species. The leaves help to acidify the water and create hiding places for bottom dwelling fish (ie banjo catfish). The best leaves to use are beach or oak leaves though i believe the leaves from any deciduous hardwood tree are suitable. Collect the fallen leaves in autum (fall) and dry them out to be stored, when needed boil the leaves until they sink. The leaves in the tank will need to be replaced when they look a bit rotted.