If you caught it early enough, there may be some hope. Change the water and add 1 tsp of salt per gallon of water. Or go to the pet shop, there are medicines for it, but if it's been going on for a few days, it probably won't help. If you decide to give medicine a shot, look for an Internal Antibiotic as external ones will do nothing.
It's usally caused by dirty water and a poor diet.
I hate to say it, but don't be surprised if it dies.
How big is the aquarium and how often does she change the water?
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