I have a 125 FOWLR populated with seven butterflies, most have been with me for over a year. The tank has been running for three years. In the last month, I have lost two long term fish with the same symptoms, head shaking and weight loss. Up to the day they died, they were active and eating well. I thought possibly flukes and treated three times with Prazi but still lost them. Of the remaining fish, one is showing the same symptoms. The other fish are symptom free. Any suggestions?