New Tank, in cycling process, fish dead on day 2, what next

I'm new to owning fish and I have no idea what I'm doing! We bought a 5 gallon tank a few days ago. We rinsed it out, wiped it down, filled it with already rinsed gravel and decorations and about 4.5 gallons of distilled water with a dose of stress coat. Then, yesterday, which was 24 hours after setting up tank, we added one freshwater plant and two glowlight tetras. I did exactly what the fish store guy told me to do and I planted first, and then put the plastic bag with the fish in the tank for 15 to 20 minutes before releasing. After researching online I did note that I made one error in that I dumped the bag into the tank after the 20 minutes instead of netting my fish out of the bag so I did get fish store water into my tank. But, with that said, the test strip still says it's at ideal condition this morning but one of the fish has died. Is this just part of the cycling process or a stressed fish? What's my next step? I only have one fish in this tank with a plant. Do I add more?

your tank sounds like it is set up fine. the fish you chose to add right away probably were not the best choice because they are a very sensitive type of fish, also acclimation (puting the fish into the tank) wasn't done how it should be. a lot of people overlook the step of slowly adding your tank water into the bag that is floating in the water with the fish in it. when you are acclimating you are not only slowly adjusting the temp you are also slowly adjusting the ph and all of the other water conditions of your tank. acclimating wrong will kill fish, its too much of a shock for them to go from one type of water into another without time to adjust.