Pro Shrimp's comment: Alder Cones - very similar to Indian Almond Leaves - release tannins into the water. Acting antibacterial and stress relieving for the inhabitants. Alder cones are very popular with baby shrimp to graze on. Additionally they are a very attractive addition to any tank.
Alder cones can be added straight to the tank where they will be floating for a while until they sink to the ground. To speed this process up you can boil them for a couple of minutes as well. Over time they will slowly degrade and can remain in your tank.