A large single piece of slate would fit nicely into a Amazon biotope tank, large smooth round river cobbles could also be used but the main decor should be wood.
Oak and Beech leaves can be used for leaf litter biotope tanks. Collect the fallen leaves in Autumn, dry them out and store them in a cold dry place in black plastic bin bags. To prepare them for use take enough leaves to cover the bottom of your tank with a 2" layer and boil them for around 5 minutes. It is difficult to vacume this type of substrate so stocking levels should be low to maintain water quality.