We went with what was available. Sometimes mice and sometimes rats. A rat of the same size as a mouse is heavier meaning more nutrition in a smaller package. We also had a super hard time getting a regular supply of mice locally for some reason. The zucker rat thing... I did a ton of reading on this. It's not about feeding pups/weaklings/small/medium/large rats. It about the jumbos. The old fat rats were causing the problems. You can avoid this by staying away from those jumbos. Also, there are so many colored rats out there that aren't Zucker's.