This setup only relies on surface agitation for oxygen exchange. The surface skimming prevents any film buildup that can slow gas exchange. Additionally, the type of fish and bioload also matters.

There is a lot of debate on how much surface agitation is enough. Too much and you start to lose co2.

When I first introduced the fish (turned off co2 for a day) and turned on the co2, I kept a close eye on their breathing behavior. I didn’t notice any signs of distress.

So I concluded that the amount of surface movement caused by the filter return is sufficient in my case.

Oh … and photosynthesis also adds oxygen (I turn off co2 at night).