Hmm. If no ammonia or nitrites, then I wouldn't suspect that's the cause of fish death. I wouldn't rule out that your tank may cycle in the next few days, but with 60 lbs of rock you should have a fair amount of bacteria on your rock and in your sand to start with. It's not like you're starting completely over again. I'm assuming you had some type of media in the cannister like sponges or ceramic rings or something like that?
Only other thing that comes to mind is oxygenation. By removing the cannister, you removed a good chunk of circulation. But then I'm assuming your return pump from your sump is probably providing more circulation than you took away?