As anyone who has ever snorkeled or gone scuba diving can tell you, visibility under water isn't anywhere near as good as it is on land, due to the denser medium. So they can't actually see that well, but they sure do have a great sense of smell. In addition to nostrils, all fish have an organ that runs down the side of their bodies called the lateral line. It serves as a combination sense organ, picking up the 'taste/smell' of chemicals in the water, as well as enhancing their sensitivity to vibrations.
So yea, the smell of food attracts them much more than seeing it. Drop a Hakiri sinking wafer on the other side of the tank from a bunch of corys, and see how fast they find it.
Of course, that's not to say that they DON'T use sight to find food -- if they're close enough to it, they'll see it floating in the water (see the feeding time frenzy for details

). It's more that they use a combination of senses when the food isn't quite so obvious.