I recently had a similar experience, but the water was milky white. I came home from work, and when I turned my light on the tank, it looked like mass murder. My parameters appeared normal, and after many possible reasons for the deaths, the most logical one for me was that a gas pocket in the gravel had been opened, and the stuff in the pocket poisoned my fish. I have parts of my tank with rocks that don't get throughly cleaned every week like the rest of my gravel, and these areas apparently get a build up of bad stuff that turns into poison if it's not properly aerated on a regular basis. I wondered how people with heavily planted tanks avoid this since they usually don't deep vacuum the gravel, and it was suggested to poke the planted areas with a knitting needle at least once a week to keep it properly aerated.