Why is my guppy at the top of the my fish tank?

That agitation counter-question is interesting because fish will gasp at the surface if the tank is low on oxygen. UnfortunateIy water with too many pollutants (mostly ammonia products() can cause that. In fact the top 1/2 inch or so of the water is richer in O2.

However it is very natural (if there are no predators around) for guppies to cruise the surface. While they can take food anywhere in the water column, from plant surfaces and from the tank bottom, one can notice that their mouths are slightly up turned. This makes it easier to feed from the surface.

They wil take small insects, larvae like mosquito larvae, small crustaceans drawn to the light and even all sorts of microscopic stuff. A professor friend maintains a 15 by 15 foot "pond" in the hall of the science wing at a local college. It is well planted. He adds a few tilapia (an Oreochromis mozambiqus sport) to eat hair algae. I was surprised to see guppies in there as well. Most of them are tolerated by the well fed tilapia (don't try this at home though) and their purpose in being there is to eat the occasional skim of whatever organic film is at the surface.

The guppies are not along in that habit. One of the nick names for livebearers and the rather closely related killifish is "top minnows."

how much surface agitation is there?