Once you have proven that you are a talking fish it is no longer a matter of belief, but of acceptance.
There is no way to not "believe" the truth. It is possible to not accept it, however. So this question is not only misleading, but pointless- it answers itself.
I'm a talking fish myself, I think it's awful that some people would disagree with such a motion. We are what we are, don't repress our existence, we only want to communicate. It's like Martin Luther Trout said ' I had a dream, that fish and people would live together in harmony and acknowledge each other's existence.
If you claimed to be talking to a fish, I simply would not believe you. It would be an unsubstantiated claim. If I saw you talking to a fish, there would be a significantly higher chance of me being schizophrenic than you actually talking to a fish. So no, I would not be believe it.
As you can tell by the picture, the t-rex is clearly doing the world a favor by eating this dreadfully elusive fish. It therefore couldn't possibly exist. The dinos ate 'em all! If it turned out I were a talking fish, I couldn't possibly be a talking fish. That's just crazy talk. Nobody should believe in things that turn out to be true. Everybody should stick to their guns and never back down, 'cause that's how the world works. Even if all reason and evidence contradict a belief, say it's magic! We just gotta believe guys, and have faith that I'm a talking fish, and then reject the fact that I'm a talking fish. By the way, this is a magical all-powerful wish-granting-if-it-feels-like-it talking fish. He should totally be worshipped.