I hope you have gotten a small air pump, to add to the tank, that is to help aerate the tank, I have a strong suspicion this was the problem causing them to die before from insufficient oxygenation of the tanks water, that is such a common problem.
A couple of other problems that can arise
1 of those can be temperature shock, when changing the water of the tank when you do a tank cleaning or from when transporting new fish home from the store, and into their new tank. Sometimes the water temperatures differ and the fish being transported in do not have sufficient time for the bag they arrive in to adjust to the water temperature they are being moved from and into this can sometimes cause the fish to go into shock, which can easily kill them.
1 way to alleviate this is to place the bag they arrive in into there new tank still sealed somewhere from 30 minutes to 1 hour and allow it to float around for it to adjust to the tanks temperature before releasing them in to your home tanks environment.
Since I forget the exact amount of time they should be set to adjust to tank temperature, Ask your Fish shop for more information about this also ask the shop for more information about proper storage of the fish when you do tank cleaning & for transference instructions when moving the fish from the store's tank to your home tank please ask the shop for the actual time the fish should or can be stored in the bag while cleaning and transference from store tank to home tank
Another problem you may find with long standing fish tanks is stagnation of the water & of the tanks side building slime aka getting dirty, there is further information available on that, if you wish for me to find some links, just ask I will see what I can dig up.
Now As said before best of luck to you and your new fishes.. Such wonderful little critters.
