The amount of dissolved oxygen in a tank is determined by the total air surface, not the filter. The filter merely moves the water around and eliminates small bits of debris. Also, the bacteria in the filter actually consume oxygen, not add it. By circulating the water, air stones will slightly increase the oxygen content, but they are not necessary to maintain fish health.
A correctly-stocked tank allows ten square inches of air surface for every inch of fish. A tank 30L x 20w would have an air surface of 600 square inches, which means it could support about 30 two-inch fish, no more.
As previously stated, the most common mistake of aquarists is putting too many fish in a tank, which then requires excessive filtration just to support the overpopulated aquarium.