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.
Bubbles created by agitation effectively increase the surface area and aid in diffusion. Air stones effectively do the same thing.
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.
Wow, I thought the inch per gallon rule was bad... There are WAY too many variables to make a statement like this. With this, In a 20"x10" (Standard 10G) you could support 30" of fish. I stock 70+ tanks heavily and that's absurd even to me.
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.
You made a statement of fact which simply isn't one. IMO (and only that because I haven't conducted any polls or studies) the number one mistake for aquarists is improper feeding (too much, wrong type, no variance, etc.).
I really don't want to seem argumentative here, in fact I like this discussion. It seems the forums are somewhat void of discussions most of the time. We all have our experiences and opinions. I only chimed in because (once again, IMO) you gave bad advice to the OP (re:shut the filter off at night) and it was the OP I was concerned for.