wood will not release tannins for long. if you use peat, you had better pay very close to your water because it will effect your ph. peat must be changer frequently or the ph will rise again. I have wild discus in a "blackwater" tank without the blackwater. tannins are not harmful to any fish but are certainly not needed. blackwater is the location of certain fish and the waters are usually tannin rich. and no, you don't need carbon except for removing meds. carbon would also remove the tannins feom the wood or peat. you can also use the blackwater extract but with a large tank it would become expensive as you would have to replace it with each water change. here ia a pic of my tank. lost the tannins long ago even though there is a lot of wood.
