Contrary to what people think, mantis' are not the easiest critters to keep. They have a habit of burying their food and thier molt which hurts the water big time. They are very sensitive to temperature and salinity changes (like any other shrimp).
I would venture to say that it wasnt necessarily just the powerhead. Maybe there was something else brewing, and the stagnant water overnight just kicked it over the edge.
That really sucks. I had one that just died one day (he molted a few weeks prior too).. and the tank he was in I still have, and its perfect.. who knows
Good Luck with your next one (I hate to say that)
Jess