I had this happen when the main water pipe in front of my house was replaced about 10 years ago. Immediately afterwards, I lost two tanks of discus fry, two breeding pairs and a ton of angelfish. After a week of losses, I went to a 100% R/O system and the problem went away.
Something is added to the water to seal new pipes. I don't know what it was and how long the treatment last. I ended up relying on R/O for 3 - 4 years before going back to tap.
Willie