I had a similar situation happen with my tank also when it was around a month old and the same thing would happen, the fish would pretty much die over night and for the life of me I couldnt figure out what could be killing them. I also thought that perhaps there was a chemical in my tank but then it dawned on me, after much reading and research, that my tank was just too immature at the time to handle any more of a bioload and would cause my tank to have a mini-cycle. I then preceeded to wait out another month before adding fish and that seemed to help out alot. I have had all my fish alive for just over a month and they all seem to be thriving now. Good luck with your tank and happy reefing!
