Do you used tap water in your water changes? Try r.o. Some of the mystery stew that ends up in the public water supply can only be found with extensive laboratory testing. It's hard to even get a straight answer directly from the water utility. I'm in Florida and our water supply is erratic, to be polite. Sometimes you can smell the chlorine, other times it tastes like pure rainwater. All our water changes are r.o., finally going to hook up an r.o. unit to the kitchen sink and plumb it back through the living room wall to the tank, I'm tired of buying water and carting it home.
Had a friend lose a 200+ gallon salt (fish only) years ago when the local water dept. put something scary in and no one knew...until he did a water change and crashed the tank.
Good luck...keep the hate pure