To be honest I don't think there is much decreasing in tension in this short story - it gradually raises the tension until it reaches fever pitch at the very end as the narrator declares himself for who he really is and the terrible crime he has committed.
From the very first paragraph, that makes it clear we are presented with an unreliable narrator, we are plunged into a scary world of imminent violence and madness. Every paragraph raises the tension another notch as we wonder what the narrator will do and then think about what will happen in consequence of his actions. Consider the second paragraph:
Passion was there none. I loved the old man. He had never wronged me. He had never given me insult. For his gold I had no desire. It was his eye! yes, it was this! He had the eye of a vulture - a pale blue eye, with a film over it. Whenever it fell upon me, my blood ran col; and so by degrees - very gradually - I made up my mind to take the life of the old man, and thus rid myself of the eye forever.
We are presented almost at the beginning of the story with the plan of a narrator who we suspect of being mad to cold-bloodedly kill an acquaintance for what seems to be the most ludicrous of reasons. How he does it, the reaction of the old man and what happens afterwards only serves to keep the tension rising in this rapid and terrifying tale.