Question
I have a mature, well filtered and balanced pond. I keep it lightly salted for the health of my koi. This year the water has turned a tea or tannin color.  there is no mulch, wood, or leaves in or around the pond.  I have an ample filter, UV light, and add bacteria.  Still a tea pond!

jim

Answer
Scoop some water up in a cup.  Let is sit undisturbed for 24 hours.  If the brown color is settled in the bottom, it means that there is a buildup of iron or some other mineral in the water.  If the brown color remains suspended in the water, it is a byproduct of decaying organic matter, such as plants.