Oh, have there been any water problems?
Do you have the water parameters for the tank?
Ammonia, nitrite, nitrate?
Doing some water changing seems like a good start.
, then another, and another. The first 2 one then another within a couple hours. 3rd around 6-8 hours.
It looks like it could be a bacterial infection. Clean fresh water is the start to helping quite a number of fish issues. Aleays match the water temp an use dechlorinator. Prime conditioner is a really good option.