I agree with Navarre. A filter and or an airstone allows you to load the tank above a natural level. Cosidering that you can get over a million bacteria on a pinhead, there will be heaps of nitrifying bacteria in a tank without a filter. Having a filter encourages more bacteria because of the continuous flow of food (until most people over clean it). An airstone increases the surface area and allows some overstocking of fish. An unfiltered, unaerated planted tank is fine provided it is not over stocked with fish. I have a number of unfiltered, unaerated and unplanted tanks that are lightly stocked with fish and they are fine.