Addition of Canister Filter==> Fish LoSS pls HELP

You probably exposed anaerobic bacteria to air by stirring everything up and adding alot of new (highly oxygenated) water to your tank which probably killed the bacteria. Without that bacteria, the ammonia that your fish produces was not being "processed" so you had a small ammonia spike which lead to a nitrite spike. Expect to see a small spike in nitrates some time soon. As long as you don't do anything that drastic again, you shouldn't have to worry about that happening again.