Hi, welcome to AA!
To help diagnose the problem, there are a few things we need to know:
What size tank is it?
What kind of fish did you get and how long did they live?
Are there any fish that are doing ok?
Did you cycle the tank before adding fish?
Do you have LR
in the tank and is it cured?
I would also recommend getting a test kit so that you can properly monitor the levels in your tank.
Water params would also be useful.
What kind of flow do you have in the tank?