That is a very nice set up.
Don't be too upset about a few losses. No one does everything perfectly the first time in this hobby and as Brad said you could have had unhealthy fish to begin with. Sometimes they look fine in the store but aren't eating well or have parasites, etc.
I'm no expert or anything but it looks like you're off to a great start. Welcome to Canreef
ETA: Cardinal fish are rather sensitive. Not sure if yours were captive bred or not but try to look for the CB if you are going to replace them. The CB ones tend to be a bit more hardy.