Okay, I have a metal water dispenser on a heating plate. Is it okay to add mineral and vitamin supplements to the water in a metal dispenser? And what additives should I avoid using in a metal dispenser?
First, you should always have the option of fresh, clean, unadulterated water available for your birds.
Second "metal" dispenser doesn't tell us much. What you can place in aluminum, what you can place in tin, and what you can place in steel differ somewhat - steel with its quality and treatment (there are grades of steel).
Generally, avoid acids and many to most salts - though concentration of salts is a huge factor in that equation. Likewise w/ strength of the acidity. and if you have two different metals in contact, that's a whole 'nother issue.
WHich leads to the third and final question. Why do you believe you need to put anything in the water?