I had a Red Pacu which was about 9 inches in length in a 150 gal aquarium that suddenly developed a milky haze over its' eyes. Within 2 days the fish had expired. After a 3/4 tank water change and after a month I had added another Red Pacu, within 8hrs. this fish has the exact symptoms of the latter. Both eyes are now milky to the point the pupil is not visible and the fish is listless. The pH, ammonia level and the nitrate levels are fine. What would cause this? Bacterial infection? Metals? I have been keeping fish for more than 30 years and have never experienced anything like this.