Welcome to the Avenue!
Do you know the age of your IRN? What was his previous living situation?
You may want to work on redirecting your bird's aggression through positive reinforcement, such as training or ignoring bad behaviors and rewarding the ones that you want. I would recommend the book
The Parrot Problem Solver by Barbara Heidenreich if you haven't read it already, as it addresses many of the problems that you are mentioning and is overall an excellent read for any parrot owner.
Usually, parrots do not bite randomly. Try to record your interactions with your bird every time that he bites. What were you doing at the time? Were other environmental factors influencing his behavior? What was your cockatiel doing before or at the time of the biting/attacking?
Most Indian Ringnecks give plenty of signs before they bite or attack, such as eye pinning, spreading their wings or tail, vocalizing, and/or raising their feathers. Of course, these can happen only a few more moments before the "attacking" occurs and so can be difficult to predict.
If your bird is unable to be handled, consider using a t-stand or a perch instead of your hand.
Here are a few more resources:
Aggression and biting is very common with Indian Ringnecks. Biting usually happens during puberty, 2 years of age. The best way to deal with this situation is to train your bird with patience and...
indianringneck.weebly.com
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Learn about the colorful Indian ringneck parakeet, including information on its temperament, care requirements, and more.
www.thesprucepets.com
The attributes listed in this thread are the experiences of each individual and your experiences may differ with your individual bird. First, THE GOOD! I have had Keiba for a few years now, and my experience has been nothing but joyful. Keiba has a friendly personality, and he is very...
forums.avianavenue.com
Training is probably a good place to start for your particular situation though.
Best of luck to you and your feathered friend!