So I bought a few mice a month ago from Petco, and they all ended up dying after their 15 day guarantee, so I had to buy a new batch of them.

Just last week, I bought 3 females and 1 male. To my frustration, two have died including the sole male and this time, I went back to Petco and got them replaced.

I'm not understanding what could be going wrong. They are placed in a 10g tank, with food and water daily... so far I have noticed they are very active upon arrival, then they get lethargic and start to look thinner and their food pellets are just being ground up and are sleeping on it?

I haven't bred mice before, I also have rats and they are being taken care of the same way and fed the same diet - but haven't had fatalities with the rats.

Any help is appreciated.