If they are dying its most likely your tank,
Make sure you:
-Have your tank cycled (1-2 weeks before you add fish)
-Your filter is working properly and you do weekly water changes while syphoning food and whatnot off the bottom
-Maybe you havent introduced your fish to your tank properly (Ill explain that @ the end of the post)
-Or possibly they are bought from a bad breeder or fish store
To introduce your fish to the aqaurium
- Bring a styrafoam box with styrafoam cheese curls in it to reduce movement and place the fish carefully inside after purchasing the fish in the store.
-Then make sure you take them right home to reduce the stress and loss of oxygen. make it dark but have the slightest bit of light
-When introducing the fish to the tank, float the bag in the aqaurium for about 20 mins so they get used to the temperature. Then open the bag and take a small amount of your tank water and place it in. Continue floating the bag for about another 10-15 min.
-Then, slowly pour your fish into the tank and again, make sure there is little light as possible
-Allow the fish to stay in the tank with no light for about 5 hours or so, then with dim lighting, place some food in the tank and allow the fish to eat.
Whala there you have it.