A sad, sad day for FishyFishy

[naesco]

Sorry to hear this.
When almost all fish die suddenly in a tank it is often oxygen starvation. This can happen for a number of reasons including a combination of a tank overheating, sand bed disturbance, too many fish for the size of the tank and lack of flow near the top of your tank.
Did you observe that the fish were swimming near the top of the tank prior to leaving your home?