I just bought my children 6 goldfish about 2 weeks ago and Lucky isn't turning out to be so lucky. She (or he) has a black spot on her top fin and 2 on her back fin. She seems lethargic and sometimes floats on her side. I removed her from the 20 gallon tank where the other 5 fish reside because she kept floating into the air bubbles and was being thrown around by them. Does anyone know what is wrong with my fish? Is there anything we can do for her?

Thank you,
Amy