Why did my clown fish die after 1 day in aquarium?

I have had my new saltwater tank set up for almost 2 months now. It's a 14 gallon BioCube. I have about 10-14 Lbs of live rock in it. Also have a flourishing Xenia soft coral, 3 snails, 1 turbo snail, and 3 hermit crabs. I had 2 damsels in to start the cycle but both of those died after about 2 weeks which I was told is normal. After getting my water tested 3 times over the course of 6 weeks my local pet shop told me it would be OK to introduce a fish again. Parameters measured by him were Nitrates, nitrites salinity, and PH. They told me everything was a go so I bought a clown fish for $10. Used the method that the shop told me. Let the bag float in the water for 30 minutes. Then take it out, open the bag and pour a little of my water into the bag 3 times. Once every 7 minutes. After that I dumped the fish into a fish net over a sink and placed it into my tank. The fish seemed very hyper and kept looking at everyone outside the tank. Woke up the next morning and it was dead. What are some things that I may need to look into as to why the fish died? Do they die just from stress of a new tank and can't handle it or could a parameter be off?