I set up my tank (I haven't added my fish yet), and at first the water was fine and clear. Yesterday when I looked at it, the water was cloudy!!! But the only thing that I've done with it since I set it up was take out the ornament that came with it and put in a mug (my fish has a gill that sticks out, so I'm afraid he'll get injured on the other decoration). The tank has a filter in it, also.
Does anyone have any idea why this would happen?