Re: danio have got something on it's side.... is this emergency?
The most likely is the start of a fungus infection which melafix is usually a good treatment when the problem is caught early. If the problem has been there for more than a week i would suggest using an anti-fungus treatment as it should be more effective.
I would highly recommend having the aquarium water tested to see if that may be the root of the problem, also unbalanced water can have adverse effects when fish treatments are introduced.
The most important thing is to have your water tested. Goldfish are generally very hardy fish and it is fairly rare to have severe problems with them. Normally if a problem occurs it is due to a water issue and can normally be treated relatively easily with chemical treatments.