My 40g tank went from cloudy/milky to brackish, or more of a yellow tint...Can anyone help me?

The tank was recently set up, and only holds ONE goldfish which is about 2-3 inches long. It is one of the tiny feeder fish you can get for less than a dollar. It has no live plants, and the decor is same that was in its original 1.5g tank. The water went cloudy a day or so after setup, and I waited a wk b4 using a store bought treatment used to clear. About 15 min after the treatment, the water turned dark brown, almost like river water, u could barely see through it. It has since lightened to a yellowish (bout a wk later). The filter is a carbon insert type that says it should handle a tank of this size, however it is old. Gary (the fish) seems totally oblivious but i would like to stay away from using more chemicals if possible. Is this something I have to just wait out, or are there things I can do to facilitate clearer water?? From other questions Ive read, I now know it seems the tank had not cycled, and i jumped the gun. Can anyone offer any advice on what to do?