Eddie Fencer’s arm was surrounded by piranhas. They circled menacingly at first, one or two occasionally coming forward and toying with the idea of tearing a dime-sized chunk out of his flesh. Eddie’s hand made quick, lively movements and the silvery-red fish scattered, terrified.
He pinched the dull end of a razor blade between his thumb and index finger, wearily scraping algae from the inside of a large aquarium.
Eddie was careful to always move quickly but deliberately so as not to scratch the glass or knick the silicon edges. The piranhas were all huddled in the corner, now terrified of his gently swaying arm. They can’t see above the refractive surface. As long as you stuck your whole arm in and not the tips of your fingers, piranhas are cowardly little fish.
 

* I want to know he's cleaning the aquarium *before* I know about his arm among pirhanas. I was picturing somebody's arm, cut off, and thrown into the Amazon.

* I'm careful of discussing feelings of boredom, weariness, etc. -- you can do it, but you run the risk of having the reader feel bored and weary (by suggestion) and quitting the story.

* I want to know in this snippet why we'd want to read the story -- what it's about. I've cleaned fish tanks (not with pirhanas), and I didn't find it exciting!
 

And, well, I don't know if this matters much, but all 3 paragraphs start with "Eddie" or "Eddie's". =/
 

Switching to second person here pulls me out of the story. Substituting "he" and "his" for "you" and "your" is consistent with your first paragraph(s).