Food needs to be kept available at all times. Laboratory pellets (Lab Blox, Rodent Chow, Pet Blocks, etc.) as the main diet, supplemented with a good quality grain mixture given in small amounts a couple times a week, is the best basic diet. You can find these in pet shops or feed stores bagged in small quantities.
If lab pellets are not available in your area (you may have to ask the store owner or manager if they can get them if not normally stocked), then a high quality dog food (not over 8% fat content) from the pet shop/feed store such as Nutro, Science Diet, Iams, etc., fed equally with a rat/mouse grain mixture is a good substitute.
Complement either diet with small amounts of salad greens (clean, freshly washed, non-contaminated or sprayed, dandelion leaves can be a treat for mice), fresh fruits (rats love bananas, also avocado given in small amounts) and vegetables (raw broccoli and corn-on-the-cob are a favorite with rats) (NOTE: any fresh foods should be washed when necessary), and whole wheat bread. Be sure to clean out any uneaten fresh foods the next day. Be sparing with oily seeds and nuts.
Dry cat food should only be given to growing youngsters or nursing mothers because of the high fat and protein content. DO NOT give your pet treats such as candy (chocolate can’t be digested by rats), cookies, potato chips, or other junk food.
Treats such as dry, healthy, low-sugar cereals (Cheerios, puffed wheat/rice/millet, spoon-size shredded wheat, etc.), plain popcorn, wild bird seed, dry oatmeal, occasional table scraps such as veggies, salad, spaghetti, etc., are okay and will be eagerly devoured by your pet.
Do not feed your pet through the screen top of the cage (if the screen is large enough to do this), as they will learn that things poked in are food and grab anything poked in including your finger.