I just purchased two parrots about three weeks ago. They both were black spot free when purchased. The small yellow one started to develope black streaks on back and in dorsal fin in just a four day period of time. I am pretty certain that this "disease" is stress related. This yellow parrot developed the black steaks when I went away for the holidays. I had my brother coming over and feeding them. Even though they are not really comfortable around me and my family, they really get nervous and hid when anyone else is over. The yellow parrot only comes out of his hiding places when I sit in front of the tank and stay still. He has started to associate me with food, as well as my other parrot. About four days after being put on his normal routine, the black streaks and spots disapeared. During this time I also started adding more harding salts, like magnesium and calcium. Parrots seem to do better in water at the middle or higher end of the hardness range of 5-15 degrees. I now keep them at 11 degrees, most of which is calcium.