Two things.
Firstly... you telling someone that the petrified fish head is a mango is merely a claim - NOT an experience - and a claim I think you would find difficult to support.
If you experience the fish head as a mango (i.e. experience the sight, feel, taste and smell etc of the fish head as others do a mango) - then okay - THIS would be difficult for one to invalidate, however weird. THIS is subjective experience.
Secondly, in the example you give you would actually be delusional and should seek medical help - as one can happily subject the object to rigourous testing (i.e. compilation of evidence) to demonstrate that it factually is a petrified fish head. And as we all know, delusion is holding something as true despite being factually incorrect.
This isn't to say that what you "experience" (the taste, smell, feel etc) is invalid - only that the "experience" is most likely based on abnormal brain-function and could lead to incorrect conclusions whenever the false assumption is used. Which can be dangerous (experiencing a continuing walkway when actually there is a 100-ft drop, for example).
So you might believe you "experience God" (or the presence of God, or whatever) - and this is your interpretation of the very real (presumably) "experiences" that you have - just like believing the fish-head is a mango.
For you to make the claim that it is God you are experiencing, however, and expect others to accept it, requires more.