Things that kill quickly would be toxins (dino die off possible or sensitivity to added medications), oxygen deprivation (from added medication or die off) and certain diseases like velvet can kill rapidly (but if quarantined properly, less likely). So one question to what I think most likely happened, how much aeration does protein skimming really provide? We are keeping the aeration bubbles in a chamber flowing upwards with minimum microbubbles leaving the chamber. Most skimmers are in a sump not directly attached to the display and still minimum turn over of water and microbubbles. I would venture to guess that more oxygen exchange occurs in the display with powerheads breaking the surface than a skimmer could ever hope to achieve. Anybody measure dissolved oxygen in a tank DISPLAY with the skimmer turned on or off for comparison? So sorry for your loss, it really sucks, but just some thoughts on what may have happened.