The leaves of our common beech tree (Fagus sylvatica) are particularly valuable because they are retained in comparison to other types of leaves relatively long in the water and are decomposed very slowly as a natural decoration.
On the surfaces biofilms will grow which are a valuable food source for many aquarium inhabitants like invertebrates and fish (e.g. loricariids). Especially newly hatched fish larvae take profit of that.
Furthermore, the sheets offer numerous hiding places for young fish and shrimp.
There are no suggestions given for the amount of beech leaves per liter. As they don’t have any therapeutic effect, you can use as many leaves as you like to decorate the aquarium. 1-2 days after adding them to the tank they will sink and look very decorative in each near to nature aquarium.