Feeders usually have a very high mortality rate, largely due to the conditions they are kept in before you receive them. One of the problems with buying feeders and q-tanking them before feeding them to your fish is that they often die at an alarming rate. It is more likely just a result of any one of a million or so things that they have been subjected to before you brought them home. It's highly possible that something else is killing them, but just as likely IME that they are just dying off. Have heart though, if your females are pregnant, then they will spawn and you will quickly have more than you ever wanted. IME fish born and raised in my own tanks instead of the mass vlume LFS feeder tanks do a lot better. there will come a day soon when you will be looking for ways to get rid of them.
Dave
Dave