[Boredomb]

The tannins from the wood will not harm the fish. Some actually prefer it as that's what they live in naturally. In excess it can lower your Ph a lil over time if your Gh is low enough. Not something I would really worry about. If you want it gone you can do multiple water changes or add carbon to your filter.

 

[Boredomb]

Over time with enough water changes it come out. The only downside I can see is if it is dark enough some plants can struggle I think due to not getting enough light. Those are most high light plants through. Most low light plants will probably be fine. Tho I don't know as I have never had black water tanks.