Wow!! This sight really has taken a turn. You are all trying to help, but with the exception of ammonia, all that other info is not relevent.
For fish, damsel family especially, things like nitrates, alk, calcium, and temp are not really an issue...
What would kill a clownfish assuming it was not simply to stressed to begin with...
How long was it in the bag before you got it in your tank?
How are you oxygenating your tank?
Do you have other fish in the tank? Are they OK?
If you have other fish, it would rule out oxygen and electric shocks.
If you do not, then they might be worth considering.
If everything IS optimum, then I would simply assume that you got an unhealthy or weak specimen from the store...