There’s a lot of information on what foods to feed cockatiels - so here I’m only going to list the foods I use. If its good (not just nice!) for us it’ll be good for them.
Cockatiel seed mix - Changed every day to remove the empty shells. Cockatiels take the outer shells off the seeds before they eat them and these normally land back in the seed dish. This makes the disk look full although there can be very little seed left. (Also as they shell the seeds before eating them I don’t bother to offer them ‘bird grit’)
EMP Egg Food - This is a manufactured food bought from the pet shop that I feed each day while the birds are feeding chicks. When they’re not feeding chicks I’d offer this about once a week as it has added vitamins etc.
Veggies - Cabbage leaves, cauliflower, broccolli, carrots, cucumber, peppers and just about any other vegetable you can get. If they don’t like them raw - try them cooked.
Boiled Egg - Although this sounds a little odd. There’s a lot of good proteins in an egg, but like everything in life, too much of a good thing can be bad. No more than once a week - and I’d limit the amount of ‘white’ they have as this can build up sulphur levels. (Its the white part...okay a correction needed here (thanks Amanda!)...the white part is water that protects the egg, the yolk is a fatty food store and a germination spot is what turns into the chick - incase you were wondering! - See here for a detailed explanation - a very cool site!).
Avoid - Avocado, Chocolate, Coffee and Salty foods.
Other - Bread and toast they love - almost as much as pizza - just be careful not to feed them too much of the nice stuff.