Could you please give us more information about your tank please?
Here's a (partial) list:
tank size (I see 190l - 50g in your signature), age, amount of live rock, sump, skimmer, lighting, etc.
Water parameters and source (tap/well vs RO/DI) That means actual numbers for Ammonia, Nitrite, Nitrate, Specific Gravity, and if you plan to keep corals, Calcium, Alkalinity, and Magnesium will come into play. pH is pretty much meaningless in a marine system - I haven't tested mine in 20+ years.
Any quarantine procedure you (hopefully) had your fish in, including any medications and their dosages.
The more you tell us about the tank the better. With no information at all other than a fish died, anything would just be a guess.
Let us know so we can help you figure out the problem!