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.