Again, sorry to hear. Did anything live? I think you should stick with it, but I also think that tracking down the most likely cause is important. 300g is a huge volume for something like a critter death causing a nuke. Do you have dosers? Heater controllers that might have failed? Did you add anything recently or change anything? Stray electricity from a device? Could someone else have done something while you weren't looking?
I ask this because we had people over the house once that had a kid who was 8-10 at the time. I told him very politely that sticking hands in the water was off limits. I came around the corner not 10 minutes later and he had both arms over the side of my 60g rimless cube. I nearly swatted him myself! Needless to say, that never happened again.
Did you use any chemicals nearby? I hear that fabreeze, for example can be very toxic to marine critters. A recent water change? Do you use RODI or tap? We've had a couple instances where our tap water had elevated H2S in it - could smell it right out of the tap. Both instances occurred when work crews were doing something with the pipes near the house.
As bad as this sucks, I think some solace might be found in diagnosing the most likely cause.
