They look like Oak leaves or similar. If that's the case they act the same Almond leaves or Katapang, which are used as a natural water softner I think. People use them to create black water conditions and to help with breeding of some types of fish who prefer soft/acidic water conditions.
They will rot over time, but should last a while from what I've read
Great looking tank. I really like the Altums! Gorgeous fish!