Although there are commercial mixes available for Rats these do not tend to give the full variety needed in a Fancy Rats diet. A good option is to mix your own, using online sites such as Rat Rations given under Further Reading. A popular choice is the Shunamite diet for rats, details of which are given in the book 'The Scuttling Gourmet'.
Water
Water should always be available for your pet rats and they will take this from bottle or bowl. If the former a good rule is to have two bottles, so you have a backup if one becomes clogged or otherwise broken.
Base Mix
A good base mix is Burgess Supa Natural rabbit muesli for its variety of grains. Rat nuggets can be mixed with this as well as human cereals such as rice puffs and cornflakes providing they are sugar free. Lentils and mixed bird seeds can also be given.
Protein
Adults need a low level of protein especially as they get older. A diet high in protein has been linked to health issues. Young pups or kittens need more protein than adults however, a wet kitten food such as Applaws can be given for them.
Fresh
Rats will love additions such as Apple, Banana, Broccoli, curly kale, peas, and sweetcorn amongst others. You can also give cooked cous-cous, rice and cooked prawns and chicken. Items that are very high in water content such as lettuce and melon should be given sparingly, but are good for giving to rats in their carrier on long journeys where another water source is not available.
Treats
There are plenty of commercial treats available for pet rats as well as items such as pumpkin seeds, mealworms and sunflower seeds. These should not be given too frequently as they can lead to an obese rat.