The seeds you purchase at the pet store should be a high quality. Cockatiels do well with a variety of green veggies such as lettuce, broccoli, etc. and fruits such as bananas, apples, pears and so on. Absolutely no avocado as it's toxic to birds. Cut small pieces of the food and vary daily. Organic fruits and vegetables are an excellent choice as they are free of pesticides. You can include some nuts such as walnuts, almonds, but not often as they are very fatty. The seeds should contain supplements for your bird and with a varied diet, you don't need to add anything.