Almost all fish medications have the potential to be dangerous and/or toxic at high enough dosage rates, or if used excessively. It's been stated by at least one researcher that excessive use of metronidazole can cause organ damage in fish.
In fish, an excessive use of metronidazole can damage kidneys and other internal organs.(Bassleer, 1983)
Also, Metro applied via feed should only be done based on the fishes weight.
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 aquatuc DVM on the planet.
If I was in the OP's shoes, I'd probably be trying Clout at this point as well.
Good luck.