If you are feeding Selective to your mice for the first time, make sure you change from one food to another, gradually over 7-14 days. At first, you should mix a small amount of Selective with your current food, slowly decreasing the old diet, whilst gradually increasing the quantity of Selective.
If your mice are used to a sugary diet, you may find it takes them a while to change, but persevere as it will help to benefit their health in the long term.
And don’t forget fresh water! Your mice should have an unlimited supply at all times.