if pesticide got on your cleaning hose, then it would've been all the tanks you DID clean that had dead fish, right?
I kinda hate to ask, but how old is your brother who didn't wake you? Could he (or some other kid) have put something in the tank...accidentally..? Seems like only a real young kid would not have known to wake you.. Did you inquire about tampering, spills, etc. Kids might hope their brother would sleep for DAaays, rather than wake up to discover dead aros, especially if the kid felt bad. Those fish look like exposure to something caustic, toxins, etc with it happening THAT fast. Did you find out if an adjacent apt got sprayed?