itsme103

I have a platty who was doing just fine yesterday but today from about a half inch of the tail to the abdomen looks very very pale and almost white. From the head to about the middle of the abdomen the color is normal. Its almost seems to me like the fish can't move its tail. I had this happen to a betta once. It almost seemed to me like the back end of the fish just died while the frong was alive and well. Any ideas on what this may be or how to treat it? Any help would be great.
Thank you