I use a refractometer as well. Its a dead on piece of equipment vital to keeping salt water enviroments. I use a Sybon, was about $65. It's awefull losing fish, especially when everything seems to be running great. Did I read your on a well? If it's shared, could someone have dosed it as a normal maintenance (add bleach/disinfectant) without you knowing? I don't know any who test for chlorine/chloramines, I use Prime on every ounce of water going into my tank's. I sure hope you track it down and it wasent some random thing so you never know...Best of luck.
Life is tough, it's even tougher if your stupid.
If your not angry, your not paying attention...
150 FWLR (Eels) 75G Fresh (Barbs) 24G Cube (Reef) 10G Fresh (Beta)