I'm new to the saltwater tank world, I need to know a few things, help..?

I have a marineland canister filter running and a protein skimmer that is way big enough..I have added 30 lbs of Argonite (spelling?) to the bottom of the tank, added 10 lbs of live rock, and added my water and salt, I'm testing it every day to make sure the salt levels are right, It's been a week that it's been running, so I decided to add a few fish, but they died right after going into the tank...What am I doing wrong?