Ohh man so sorry to hear that. That is not a way to start out the day at all. Hope you get it figured out as I would have no idea what would wipe out an entire tank of different species other then maybe something that may have been on your hands and got into the tank????
Use any bug sprays, lotions, maybe took meds and had traces of it left on hands???? Try and think of anything like that since your water parameters all seem to be in check. Nitrates could be lower but will never wipe out a tank at only 20ppm.
Good luck and hope you get it stocked again in the future after a THOROUGH CLEANING.