Good old paper and pencil (OR pen) lol
I have a 3 ring binder with tabs for weight, food gender*/weight, date and if he ate or not, his weight, shed dates, temp / humidity / cleaning schedule, and a notes section.
I record weight after every shed.
*He seems to refuse male f/t rats more than female. and he strikes female rats faster and with less zombie dancing. Males I often have to brain. IDK - he's a strange snoot