Wash every new tank. Hot water, some mild dishsoap and a toothbrush should do the trick. Disassemble it completely including the O rings which can also have traces of machine oil.. I use an ultrasonic cleaner, but mainly because it's convenient, you don't require one to get a tank clean and ready to use, but it can be helpful. You can achieve the exact same results with a thorough scrubbing and rinsing, it's just more work, etc.
Anna