In an aquarium setting metronidazole dissipates out of solution in approx 8-10 hrs, so once a day every other day, especially at 125 mg/l per 10 gallons wouldn't have had any major effect on internal pathogens/flagellates. (250 mg/l is the dosage rate I always recommend)
If it was my fish, at this point I would treat the tank with Jungle Tank Buddies Parasite Clear, which contains both metronidazole & praziquantel, and is very effective against the vast majority of internal parasites, yet is easy on your bio bacteria. Start with a 25-40% water change, treat with 1 tab per 10 gallons, leave the tank lights off, and treat for 48 hrs, followed by another 25-40% water change. In worse case scenarios you can wait 48 hrs, then repeat treatment.
At least you could then rule out internal pathogen problems.
Jungle PC has saved my arse on more than a few occasions over the yrs.
HTH