Do you test your water? If you have a sickness or death you should, right away, so you can pinpoint the cause. (It would be inappropriate to complain if you are not keeping your water in good condition, since that could be the cause of death. However, fish from Walmart often can be weak due to poor fishkeeping at the store. Try to find another source before you buy more.)
How many fish are in this tank? Is it established??
It's always a good idea not to get more fish until a tank has been loss-free for a while.