hehehe fish actually breathe O2, maybe you've heard of it?
Sorry, but your statement did make me chuckle. Seriously though, many heavily planted tanks dont have issues with lack of surface agitation since the plants themselves are created a lot more O2 than surface agitation could. However you wont get that without having co2 injection, fertilization, bright lighting, and a well balanced system. Even on many heavily planted tanks there is still need for some surface agitation, especially at night when the plants are consuming O2 rather than producing it. Without this venturi, do you have any ripples or strong current at the surface of your water? Air bubler? Without injecting CO2, there is little reason to stop surface agitation. The small amount of CO2 made by fish wil be less than the CO2 that will diffuse into the tank through gass echange.
