If you are maintaining South American varieties such as tetras, angels or discus, then the addition of peat moss will do it for you. Tetra also makes "Blackwater extract" which will essentially do the same as peat moss, but faster and in liquid form. I have only ever used that with breeding discus.
Personally I always preferred peat in those type of set-ups. It tends to be more subtle and doesn't need to be added every water change.
Years and years ago they used to sell "peat plates" which were compressed slabs of peat moss, about 1/2" thick and about 8"x4", to be used under the substrate.
Not sure if they still sell those or not.
The only caveat to using peat or blackwater is it will tint the water, but that is easily alleviated by the use of activated carbon or Purigen.