help with trying to tame him?

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i have a cockatiel who is about 2 years old. we brought him off a friend without the knowledge that he is quite a vicious bird when it comes to hands. is there any possible way that we could change this behaviour. we also havent had his wings clipped because we dont believe it is the right way to go about the situation (i would like to see a vet get near him to go this anyway:]) there is also the fact that if it goes dark e.g night, then he will become very vicious and attack us. when he is in his cage at night he goes on the bottom of his cage and hiss,rock and strick at us. i dont know if this has anything to do with all of this but he doesnt like to sleep on his perches he preffers to sleep at the bottom of his cage :O. please help me.