Welcome to the forums.
If are are getting any type of metallic taste there only two reasons come to mind.
One, you are washing out a non-washable hose after you use it. (very bad)
Two, you have a common chamber hookah that has rusted inside. (worse)
Either way, if you have a 10" hookah and TBH I have never even seen one that small. I would seriously think about buying a new hookah and invest into something good this time. Take a look at some of the sites on here and this post below should help. Read it well my friend.