Seeing as it sounds like you are still in the weeds with this fish, if you decide to also treat with additional metronidazole to fight what is almost certain to be a case of Spironucleus, some things to keep in mind.
In fish, an excessive use of metronidazole can damage kidneys and other internal organs.(Bassleer, 1983) I am not certain as to how he defined, excessive?
Dr. Edward J Noga suggests:
1. Bath
a. Add 19mg/gallon and treat for 3 hours. Repeat every other day for 3 treatments.
2. Prolonged Immersion
a. Add 25mg/gallon and treat once daily for a total of 3 times.
b. Add 95mg/gallon and treat every other day for three days.
3. via feed
25 mg/Kg of body weight/day for 5 to 10 days, or 100 mg/kg of body weight for 3 days. Retreat once if required.
Dr. Edward J. Noga, MS, DVM, is a highly respected professor of aquatic medicine and immunology, that has been published approx. 150 times in related papers/journals. His lab at NC State University specializes in the study of infectious diseases of finfish and shellfish. His book, Fish Disease: Diagnosis and Treatment is the go to "health" book for every serious fish keeper and aquatic DVM on the planet.