After that nice build up from Stanley, I'm sorry to say at first thought I can't think of any Lewis story like the one you describe, Larry. Lewis does use the image of fish in a tank a couple of times in Miracles, but I don't recall that specific usage.
Now, Philip Yancey tells a story like that, in his book The Jesus I Never Knew, in the chapter called 'Birth: The Visited Planet'. Is it possible that his book is the one your thinking of?