My bowl (when I used it) self-cycled with 100% water changes as well. This is before I even knew what cycling and ammonia were. I simply bought a bowl, some gravel, and a java fern then threw them all together. For the first day the tank was cloudy but after that it went clear.
Weekly I would pull out the fish & plant, put them in a 1g jar of new water, wash the tank with hot water and eco-soap, rinse the gravel in boiling hot water then reassemble. My theory? Nitrifying bacteria will fix to the roots of plants. This is probably how they traveled into my tank. Since I never rinsed the plants, every time I washed them all out of the gravel some of the survivors from the Java-Fern roots would re-colonize the tank.