How did you cycle the tank? I'm not sure about the biospira product but is doesn't sound right to add 7 fish at once to a new tank. You said you checked the parameters weekly, so has the tank actually gone through a cycle? Also, I'm curious about what happens in the office over the weekend when the fish die? Any cleaning services using chemicals? Does the temperature change dramatically? Any power outages that you might not know about? I remember having a tank in a basement and the power was connected to the light switch and I didn't know it! Everytime I went down there (I turned the lights on from upstairs) the filter, heater and lights were fine. When I went out of the room (and turned the lights off) everything stopped. It took a while to figure it out. Just a thought.