I'm not going to nay-say because this is a great concept and I'm glad you brought it up. It is probably going to make a difference in many people's filtration techniques.
But I have to ask:
If there were ammonia in the center of the tank, wouldn't they avoid that area? Just like trout rising and falling with temperature changes. I mean, if part of my room smelled like butt, I would avoid it. So can we watch the areas in which our fish roam as a clue, as well as testing the points of stagnation?
You know what?! Imma take a turkey baster to the tank and get a sample from the least-circulated area and see what's lurking. Think it's only purposeful to test for ammonia? What about nitrite, etc?