Cory cats rise to the surface to gulp air, then store it. It may be an evolutionary adaptation due to the low levels of oxygen in their native waters, which are warm backwaters of So. Am. rivers.

Plecos will occassionaly do the same, but if you see the behavor often, the oxygen levels in your tank water may be too low.

If in the least doubt, water change.

Okay, I just did my monthly 25% water change, and the pleco seems to stop darting to the surface. However, just in case, how do you raise the oxygen levels in a tank? Just in case he starts doing that again.