What he/she said. Water changes carbon, and purigen in your filter clear tannins. You will continue to have them until whatever is leeching them is all out and stops. Sometimes it takes weeks, sometimes longer. I have a large piece of malaysian driftwood in my 75 gallon that still turns the water tea colored now over a year after I bought it. I do weekly 50% water changes just to keep the tank viewable. My fish sure love the consistently good water quality.