I try to always age my water atleast 3 days before using. I also run it through a GE smartwater filter first and use either Prime or Amquel. Bichirs are surface breathers so oxygen levels wouldn't have anything to do with their loss. The last time I didn't age my water properly I lost 4 f1blk calvus. Most cities will tell you what and when they add chemicals to the water supply, all it takes is a phone call. Sorry to hear about your losses, and hopefully you'll be able to keep it from happening again.