A friend suggested an additive called Blackwater. Would this contain buffer, or do I need to get a more concentrated version?
Blackwater Extract contains trace elements, vitamins and extract of peat. It replicates blackwater conditions by creating clear, “natural”, Amazon-biotope conditions in the aquarium. Blackwater Extract promotes fish activity levels and is an excellent conditioner for all soft-water fish including discus, angelfish, tetras and killifish.
The drift wood or peat can help lower pH but they are not a "buffer." In an aquaponics system where you are not doing extensive or regular water changes, the action of the bio filter long term will bring system pH down over time if there is no buffer in it.