ouch, you have my condolences. We have all been through these situations, but with that 12" dat and arows, I would be crying! The worse it that you have no closure until you find the COD. Maybe the bb died in the gravel, and that killed them. Or some contaminant when transferring them? Hope you find out, as well as recover from this. 
both tank run with filter and power head, not sure why wouldn't be enough oxy?