People have different opinions and experiences, I'm just sharing mine , it's always good to crowd source .
Something is definitely up health wise, be it a kidney bacterial infection, a gi bacterial infection, a yeast overgrowth, or a combo, or something else.
Because your cockatiel is used to eating more than just seeds. I would suggest giving some plain Greek yogurt, must have no artificial sweetener, and should have live cultures , I like one of them to be acidophilus. A half teaspoon a day or even twice a day for about five days .
It is not going to cure anything. But it might help balance gut bacteria.
I offer from my finger and dab a bit on beak and wait. They most seem to decide they like and lick it up . After that I can put a dab down for them or still just offer from my finger..I have done this over 20 years , I always give while the have antibiotics, I do randomly here there otherwise when I'm having some. It is safe, can be found on many parrot food lists. If it freaks you out, don't do it .
I have found it helpful. I didn't have postive affects with bird pro biotics, I e tried a few. But I'm sure there are better brands I haven't. But I do see postive results when I use yogurt. I think it's worthwhile to give it a try.
Again, not a cure. Your bird definitely needs evaluated by a veterinarian and probably some treatment based on what they think is going on.
But it might help a little, especially since I think those crisp and dye fussed gi some . Possible rice fussed gi some .( note other people feed rice and feel fine about it, I just don't, it's just carbs , and I rather they get that with other foods) Not the whole answer to what I think you have going on .
Anyway if you give it a try and see some positive results, let me know..