Your tank looks so beautiful, I keep wondering on what bg to make, whether to choose velvet black or velvet dark blue for the starry sky theme. X)The brown tint from the tannins is mostly noticable when the tank light is off, when you look at the tank water against trhe backdrop of a white wall. So in other words it shoulkdn't be a problem since this piece hgas already been soaked & in some other tank anyway. Some folks even prefer the look of brown tinted water because it makes the colors appear different. Regular weekly water changes are enough to keep my tanks looking slightly stained in a good way.
I wouldn't mind a brownish water or greenish a bit, as long as no fog appears. Usually, I get a mild fog after a water change, but it can only be noticed when looking from the width area of the tank, not the length one. It settles down within an hour usually.
That new fish tank I got will some day have black sand. Still wondering what fish it will have.