There are generally two types of leaves. 1. Ones that are eaten by shrimps directly. 2. Leaves that decompose and are allowing biofilm to grow. The biofilm gets eaten by the shrimps. Drying is not a must. It is best to collect leaves for biofilm formation in the Fall because the plants absorb sugar from the leaves back to the roots, branches and trunks. If they contain too much sugar you increase the chances of bacterial blooms.   Here is a link to an earlier post.   http://www.shrimpspot