Like many things in the fish game (peat, meth blue. mal green, pot permangenate) I used to just add it until you get the depth of colour you want. Seriously, dried oak leaves are good too. The leaves lose a lot of nutrients back to the tree before they fall off so the fallen leaves will be different to vigorously growing ones. I have also used dried bark from the oak tree, and brown water from streams on the West Coast S.I. after rain which has washed through a lot of mulm to get to the stream.