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 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.
A good base mix is Burgess Supa Natural rabbit muesli for its variety of grains, this can form around half your mix. Dog kibble can be mixed with this at a ratio of 10%; good brands include Burns or James Welbeloved. Another 10% can be made up of dry pasta, with sugar-free human cereals such as rice puffs and cornflakes making up the remainder. Lentils and mixed bird seeds can also be given.
Adults need around 12-15% of their diet to be protein. Higher levels have been linked to health issues. Young kittens, or lactating does need levels more like 20-20% - a wet kitten food such as Applaws is a good source for kittens. Other suggestions include chick peas, eggs, mackerel, soya flakes and tuna.
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.
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.