I was going to post this in your other thread.
It is important to carefully identify, collect, and prepare leaves. It also must be noted that you should only collect leaves that have fallen from the trees, not fresh, living ones.
In autumn, deciduous trees begin shutting down for the winter and leaves that have worked as factories all through the spring and summer months producing sugars and oxygen gradually die off and fall to the ground. These are the leaves we want to collect because they have no living matter in them that could cause problems in the tank as it dies.