Betta Problems

I recently bought three bettas from our stores last shipment. They were very healthy looking, gorgeous colours, flaring, eating, and active. They showed NO signs of illness. Their containers were filled with clean water from an established tank, and each container had a Java Fern in it...water quality wasn't an issue. They were being cared for in the same manner as my 12 other bettas. Six days later, all of those bettas are dead, and we have been having a lot of bettas returned dead to the store.

Have any other pet store employees been having problems with the sudden deaths of bettas? I'm sure some of us must get stock from the same suppliers.