If they were healthy, maybe they died of old age and it was just happened to be like that. Have you cycled the tank or did frequent waterchanges and used something to keep them from getting bothered if you did a fish in cycle?
That's all I am thinking could have happened....though they might show signs if it was that.
It's a good idea to disinfect everything and start over.
Again, i'm sorry that happened. I'd find it hard and quite upsetting if that happened to me..
"Life is an adventure full of beautiful things, and a treasure worth more than gold."