Yep they will need plenty of meaty foods- I feed mine chopped beef heart, chopped muscle, bloodworm, shrimp brine, glass worm (all of which you can buy in frozen cubes), a good quality flake and earthworm and algae pellets. They are also partial to a bit of tiny shredded prawn when I have some! Once they are settled in with a good diet they should colour up and grow fast!
You will need to keep your water quality good with a meaty diet, so try not to overfeed to bulk them up as any leftovers will pollute your water and up your nitrates. Make sure you do a really good gravel vac and water change once a week - you will also find the water changes will bring them into breeding condition as it mimics the changes in rainfall in the wild.
Just a word of warning - if you keep them in a community tank be careful when giving the meaty foods. Unless they are chopped up really small the other fish will also obviously eat them (no harm in that) but they can get food stuck in their mouths - obviously not a problems for the cockatoos as they have such big mouths!
