The cycling is carried out by a whole range of microorganisms, each carrying out a stage in the conversion of nitrogen to nitrate and back to nitrogen in the plant decay process. It usually goes from urea from fish urine to removal as nitrate when the plants are removed after propagation. The plants can only remove nitrogen as nitrate and only when they are established and growing. All the other stages are toxic to fish and the bugs reproduce to handle the food available (in the form they prefer) and when that occurs we say the tank has cycled. A lot of the little critters that do this job for us live in the filter so the process works best when the filter is half clogged up with gunk (and helpful bugs)