If you're getting lots of traffic naturally and those that do convert end up loving your work then you have a good problem on your hands IMO. It's much easier to tweak the cover, keywords, categories, and description around if people already love your book.
Check out the listing for Battlefield Earth Alien Invasion:
The description goes in-depth about the book without revealing too much and adds in good customer reviews. Also, like crowqueen mentioned, I think the cover for the book you linked doesn't jump out as a sci-fi book. Lots of people just purchase based on the cover, unfortunately.
There's an audiobook for a dollar now of the original book, its yuoooge!
Imagine a world where fish could fly.
Come up with your own reason: Air is as thick as water, fish develop gas sacs and can become lighter than air, whatever. Invent your own rational.
Now, in this imaginary world, do fish taste like salmon or chicken?
It's your imaginary world, you get to decide with one it is. You can even write a book about it, call it science fiction, and try to out-sell L. Ron Hubbard's "Battlefield Earth".
That's the cool thing about imaginary worlds - they can be anything you want them to be!
Just don't go around making laws that make sense in your imaginary world, but not in the real world we live in.