You need to cycle a 3-gallon fish tank for about 2-6 weeks before adding any fish. Cycling is the process of building up beneficial bacteria that help break down fish waste and create a stable environment for your fish. Start by setting up the tank with water, a filter, and a heater if needed. Add aquarium-safe bacteria to jumpstart the cycling process. Test the water parameters regularly for ammonia, nitrite, and nitrate levels. Initially, the ammonia levels will rise, followed by a spike in nitrite levels. As the beneficial bacteria grow, the ammonia and nitrite will decrease, and nitrate levels will rise. Once ammonia and nitrite levels consistently read zero and nitrate levels are present, the tank is cycled and ready for fish. Regular water changes and maintenance are still necessary to maintain a healthy environment.