Oooh I recently saw a galah cockatoo on a tv vet program who had a fatty lump and the lump went down literally in a week after just a diet change! They cut out sunflower seeds from his diet so focus on the millet seeds so maybe switch to a budgie seed rather than cockatiel small parakeet as that doesn't have sun flower but has all the rest. And don't forget lots and lots of veggies! Don't bother with fruits just give lots of spring greens, kale and broccoli. Obviously exercise too. Oh and also... no human foods!!!! This galah was being fed loads of human foods even meat! Strictly bird food with minimal "treats"
Cockatiels: 1 pearl, 3 normal grey, 1 whitefaced cinnamon pearl, 1 pearl pied
Red rump parrot: 1 normal cock and 1 blue hen
Java sparrows: 6 normal, 1 lightly pied
Canaries: 1 yellow, 1 brown/orange, 1 brown/yellow, 1 silver, 2 yellow dimorphic
Zebra finches: 1 white, 1 black cheeked, 1 normal, 1 fawn
Chinese painted quail: 1 normal, 1 white, 1 heavy pied
Diamond dove: 2 normal, 1 pied, 1 white