Gas exchange is all about surface area and the amount/speed of water moving at the boundrys of said surface area. There is a few ways to do this, one of hte most common is just a spray bar splashing into the water (breaking the surface). This increases both the surface area and the amount of water flowing by that surface area to exchange gases (O2 and CO2 mainly). Another way is to have a powerhead that pushes water towards the surface, making ripples in the water (surface agitation). This is particularly effective when moving water from the bottom of the tank to the surface (or vice versa). There are many outflow attachments that will also help in this regard, lily pipes are popular around here, you just raise it a little to get that ripple effect or even some bubbles (both agitation and breakage). BTW agitation could be viewed as breaking the surface, all it means is that you "briskly stir or disturb something"
Hope that helps
Hope that helps