One large seeded sponge filter, serves to quickly start up a 20 gallon tank cycle.  An ammonia source will need to be added regularly to feed the bacteria.  Bacteria will live in the filter, on all the hardscape and the side of the tank.  Test the water every few days, and perform water changes weekly.  It may take a week or two to be fully established. When your test reads 20 nitrates, 0 nitrites, 0 ammonia the tank is ready for your axolotl or other aquatic pet.