Poorly fish

[DianeCandy]

Hello people I have fish on and off my whole life so when my daughter was born I decided to get her some fish, i have a 250L tank with 5 gold fish, they are about 3 inches long each, one of my gold fish heyhey has developed a large bubble near her tale, I have no idea what it is and the web is giving me very broad possibilites, I don't pretend to be an expert however I have got many hints and tips from fish lovers and experts so I'm fairly confident on how to keep them and how often to feed and change the water, but this has me stumped. Does anyone know what it might be or what I should do