Why is my fish dying?

A picture might help better identify the disease if there is any disease at all.

From your earlier threads, we learned your tank is too small for your fish. As well as no filter at first, then the new filter was just in and certainly it was not cycled.

When we identify a problem for "dying" fish, the first thing we look at would be testing for ammonia and nitrite in the water. These are the #1 killers for new fish tanks. Unless there is clear sign of certain known diseases.

Please provide a good photo of your fish if you want a better diagnostic.