After making a few posts on chameleonforums I have decided to not go with the charcoal layer. It will only be effective for short term, them can release toxins back into the water just like carbon. I may also hold out on springtails, or just do a outside culture for feeding babies etc. So my plan now is hydroton/hydroballs, screen, then coco fiber/forest floor(or something similar) mix with collected oak leaves on top. Don't worry, I will collect leaves at the creek away from pesticide use and bake them at 350 to kill off any hitch hikers.