Should the filter outlet be under or above water? Especially often ask this question beginners in aquaristics. There is no clear answer to this question, because how the diffuser is oriented depends on the characteristics of the tank.
Basically, however, the more the surface moves, the higher the gas exchange.

Filter outlet under water

The diffuser should be placed under water if the aquarium is not too crowded and you have many plants. Because there is little gas exchange with the air, more CO2 remains in the water for the plants to grow. To reduce the surface area as much as possible, do not use a nozzle pipe at the end and direct the water outlet against the pane, a stone or a root if possible.

Filter outlet above water

This should only be done if the tank contains many fish and has rather fewer plants, since the strong surface movement drives more CO2 out of the water and thus leaves less for plants. However, this also automatically brings more oxygen into the tank, which the fish need to live.