My tea tastes metallic
A tinny or coin-like edge that was not in the same tea brewed elsewhere.
Likely cause
Usually an unlined metal vessel. Bare cast iron and some plated novelty items react with tea compounds; stainless steel does not.
Try this now
Brew the same tea in glass or porcelain. If the taste is gone, it came from the vessel.
What to change
Steep only in enamel-lined or inert vessels. Keep bare iron for heating water, which is what a tetsubin is for, and use the lined tetsukyusu for leaf.
The reason behind it
Bare iron inside means water. Coated inside means tea.