Since I just have two, I just write everything down in a little notebook. I use it to keep track of feedings, but I also write down sheds, their weights when I have access to a scale, and any vet visits and such. It helps so much, especially in the case of my Ball who keeps having issues. Whenever I go to the vet and they ask me "when did she last eat?" or anything I can answer with 100% certainty.