When I first set up my tank, the soil I used leached a lot of tannins and the water turned yellow until after 1-2 years most of the tannins had been leached out. Now the water seems to stay quite clear for the 2-3 months between water changes/trims.
It's not harmful as you already know but you do loose some lighting efficacy in terms of what is absorbed/blocked by the tannins and not reaching plants. For your "reading pleasure", a few years back an old member did a bit of an experiment was done to measure the effect of
water changes vs. light penetration.