Fish Dying in New Tank

Question
New to saltwater tanks.  Purchased 60 gal. tank with complete setup by seller.  Introduce Damsels to tank after testing water to make sure all was correct.  Fish immediate die within 3 minutes.  Temperature is 80, salt is within spec.  Have completed changed out the water with different salt (Oceanic natural salt) to see if that was the problem.  Also changed out the filter (XP3).  Have not changed out the protein skimmer or the crushed coral gravel.  Have live rock and have been using Amquel water conditioner.  Any suggestions.  Seller says that they have never seen anything like this before.

Answer
Do not use the skimmer until the tank cycles. When you add fish do you acclimate them slowly or just pour them in? Do an ammonia test. Amquel is good but let tank cycle for a couple of weeks with the live rock and then try a couple of damsels but test the ammonia, nitrites, ph and salinity first. Make sure you bring the water in for the store to tes and do a test on their water to see if there is any significant differences, then adjust accordingly.