white mice keep dying

Question
QUESTION: What in the world am I doing wrong?  We bought two feeder mice from Petco two months ago (one plain white and one with brown spots like a fancy mouse).  The plain white one died a few weeks later.  

Then I went to another pet store and bought another feeder mouse.  Died the next day.  Then went back to buy two fancy mouse.  They only had one female fancy mouse so I bought one fancy and one feeder.  Feeder mouse died the next day.  I asked the guy at the store what I was doing wrong.  He said he didn't know that they were really easy to take care of.  

I have them in a plastic mouse cage with all the tubes and stuff.  They have water and plenty of food.  Using Aspen litter.  I'm scared these other two will die for some reason.  They are my little girl's mice.  She says it's because they're feeder mice.  
Help!

ANSWER: That's strange. It could be because they are feeder mice and they aren't really bred for longetivity but they shouldn't have any diseases as that would get the snakes and lizards they're fed to sick. The fact that they're dying could be due to a disease spreading through the mouse breeder's stock. Most pet stores in your area probably buy from the same breeder. The aspen litter gives mice lung problems but these take a while to develop and don't result in such quick deaths. Try Carefresh or Yesterdays News. My guess is it's just a disease spreading through the breeder the pet stores are buying from.
   Try looking for mouse breeders online. You'll get guaranteed healthy, friendly, fully problem free mice. At least with a good breeder. The mice only cost about 6-7 bucks if not cheaper. If you don't have a mouse breeder in your area or anywhere close to you I would just wait a month or two and try buying mice from the pet store again. Usually the stocks have been refreshed and the problems solved. I just had a thought, are these mice in an average temperature? Too hot and they will die very quickly. Too drafty and they will get sick, too cold and they will also die, but not as fast and it's actually better than too much heat because they can still burrow under bedding.

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QUESTION: Here's a thought.  Is it possible that they're eating too much at one time?  I don't remember seeing food in the feeder mice tank--but I don't know.  
They're inside the house so I think the temp is okay.

Answer
Over eating will only make them fat, not kill them. I am not completely sure what could be killing them that quickly besides a disease. Stress could be a factor. Were the mice you bought really jumpy and skittish? Because they are feeder mice they're very rarely handled and could be as untamed as a common house mouse. If you have handled them a lot then the stress of that could have killed them but that's a stretch. Did they have diarrhea? Diarrhea kills mice very swiftly because they dehydrate quickly.