The best balance for cockatiels is a 25% vitamin enriched seed mixture, 25% pellets, 25% veggies, dark leafy greens and fruits, cooked rice, corn, and bean mixture, 10% table food, cereal, bread, pasta, cheese, lean meat, (cooked, not seasoned, fatty or processed), corn, etc . . . and the remaining 15% can be proteins, berries, nuts, flowers, any treats or things not mentioned above. Seeds alone will cause a cockatiel to be deficient in vitamins.
Veggies are enjoyed by cockatiels, especially carrots, sweet potato, beets, broccoli, greens, (note exceptions below), green pepper, asparagus, ripe tomatoes, spinach, etc . . . as long as it is not listed in the never category. Veggies should be washed clean of pesticides, cut up small in a separate dish and never leave them in the cage for more than 2 hours. If possible, use a dish that wires to the cage, as cockatiels love to tip over their dishes.
Cockatiels are not going to take easily to fruits, but they should be offered now and then, in small quantities but, like veggies, should be free of pesticides, cut up small, in a separate dish, and never left in the cage for more than 2 hours. Good choices are apples, pears, peaches, pomegranates, berries, bananas, plums, oranges, kiwis, citrus, grapes, and raisins.
Proteins are important, if your cockatiels are living on a seed mixture and not pellets, chances are they are not getting enough protein. My cockatiels love hard-boiled egg yolk. Good protein choices are legumes, grated cheese, sprouts, ripe tomatoes, cooked beans, and spinach. You can give small amounts of lean, unseasoned, unprocessed meats, cooked without fat. Some people will give their cockatiels an occasional cooked meat bone to gnaw on.
Things that should be limited include iceberg lettuce, cabbage and milk, which cause diarrhea as well as nuts and peanuts as they have a high fat content and can lead to an obese cockatiel. Cockatiels prefer their nuts whole, as they love to crack the shell and it is also good to help keep their beaks from getting overly sharp. Also limit tofu, low-fat cottage cheese, hard cheese, and non-fat yogurt. Avoid seed that is high in safflower and sunflower oil as they also are too fatty.
Treats are good, but should be cut up small and limited to training only, not just given to eat for no reason or they lose their effectiveness in training. Cockatiels love millet spray; it would be best to give one about every two weeks. It is not a good idea to get your pet hooked on junk foods like Cheetos, Fritos, and potato chips, although I know people who do this, it is not good for their lifespan. Naturally, cockatiels require fresh water at all times. Never, ever, ever, under any circumstances give a cockatiel anything containing chocolate, caffeine, avocado, parsley, or alcohol.
A properly cared for and fed cockatiel will live up to 25 years so you can always count on someone to have a good conversation with when you get old. If you eat like your pet eats, you just might live long enough to get old, too!