I find it curious that people always say a small tank cannot be cycled. I have had no trouble cycling several three gallons. You just cannot overload them and they will cycle without any trouble.
Of course you have to be careful to have good media in the filter - not the little cartridges they always sell with HOB filters and the like. I always set those aside and fill with proper filter media and sponge material.
If you give bacteria enough surface to populate, a flow of oxygenated water, and a food source, they will move in and your tank will cycle properly.
I just think with small tanks no one cycles them before putting fish in, and have to do so many large water changes to keep the fish alive that they don't give the tank any chance to cycle.