I run a canister on my 55 and 125 SW
tanks. I like them for the added water volume, and a place to house extra rock rubble and to run media. If you replaced everything, there is a possibility that you may have removed some of the biofiltration when you took out the rings/bioballs.
Judging from your other post, you could have stirred up nasties from the sand bed, overloaded your bioload or when the nem died, it nuked the rest of your tank. It's really harf to tell exactly what happened at this point since there was so much going on at one time.
Don't sweat it too bad, we have all made plenty of mistakes, but you are on the right site and we'll help you get through this.