Kenny,

It sounds like a possible mini cycle along with a temp rise. The fish blocking the filter may have also increased this possibility. Other than that I don't have a clue. As long as everything was washed out and taken care of even these small things shouldn't have killed them all so quickly. But the combination of it all may have sent it over the edge. You may want to start out a little slower. Add only one at a time and even if you think the water is the same acclimate them to the new tank water. Just food for thought as I'm not sure exactly what happened.

Angie