Ketapang / Indian Almond Leaves lower pH levels and create a black water environment. It also has mild anti-fungal effects. I think it was largely used by betta breeders in Thailand, but aquarists now use it all over the world. I throw a piece in all my tanks.
Tom: what do the fish eat? Dead plants and inverts? I'm only familiar with the otocinclus that feed on algae.