Odd fish death

[fishoholic]

The top ups don't fill right to the top about a 1/2" below and it's an open top tank. The temp was 80 and hasn't changed, but I will check for electrical charges however I don't think there is any. I did think of a possible cause (not sure if this would effect anything or not but it's the only thing I can think of) we fraged our brown zoo/pally's the day before the fish died. The colony is in the main tank and we put the frag in the frag tank. Could the brown zoo/pally's (they have long stems) possibly released some sort of toxin into the tank? But if they did why do none of the other fish in the main tank seem effected?