Here's a few more reasons why you need a good air pump
What oxygenates water?
2 things :
1 -Plants -during the day (at night they produce co2)
2 -Water Surface movement. (when the surface is moved, it allows air to go into the water n the co2 to leave the water)
Now in the nature we have either plants or waves taking care of it. But in aquariums, if you don't have enough plant ratio to produce the right amount of oxygen for your fish, then you need some other source of oxygenation.
A filter will not necessarily properly oxygenate the water. You need to put the out-take close t the water surface so that it brakes the layer of stagnant water n allows O2 to come inside the tank.. Now not all filters to do that ! some barely move the surface, making the top layer of water somewhat murky n oily from stagnation.
Some people would opt for an stone. Usually placed at the bottom. But to properly oxygenate the water the air bubbles must be very fine to be well absorbed the the fish. But some air stones or anything similar to those, produce large bubbles that are inefficient to oxygenate the water inside aquarium.
This is when people begin using rotation power heads (mostly seen in reef aquarium) they brake the surface n the rotation of the pump distributes very fine air bubbles right into aquarium. This will happen only if the power head is close to the surface, otherwise you're just making waves across the tank.
But with this power head filter that i've used.. you can place it at any level of the aquarium n it will still perfectly oxygenate the tank, because the air its pumping into the water is being taking from outside the aquarium. (make sure your wire stays always out of the water.. for this to work -suction cup on wire into the outside glass works best)