Have you tested the water for ammonia, nitrite, nitrate and pH?
If yes, what are the results in numbers?
How long has the tank been set up for?
How long have you had the fish for?
How often do you do water changes and how much do you change?
Do you gravel clean the substrate when you do a water change?
Do you dechlorinate the new water before adding it to the tank?
How often and how do you clean the filter?
What are you treating the fish with currently?
Do you have carbon in the filter?
If yes, remove the carbon so it doesn't adsorb the medication.
What symptoms did the other fish have before dying?