Cycling establishes colonies of beneficial bacteria that convert toxic ammonia into nitrite and then into less harmful nitrate. Without a cycled filter, ammonia from fish waste builds rapidly and can be lethal. Two main approaches exist: fishless cycling and fish-in cycling.

Fishless cycling is the preferred method because no animals are exposed to toxic water during the process. You add a source of ammonia, typically pure liquid ammonia dosed to about 2 to 4 ppm, or decomposing fish food, to feed the bacteria. Test ammonia, nitrite, and nitrate every day or two. Over roughly four to six weeks, you will see ammonia spike and then drop as Nitrosomonas-type bacteria colonize, followed by a nitrite spike that falls as Nitrospira-type bacteria establish. The cycle is complete when the tank can process a full dose of ammonia to zero ammonia and zero nitrite within 24 hours, and nitrate is present.

Fish-in cycling involves adding a small number of hardy fish and relying on their waste as the ammonia source. This method requires vigilant daily water testing and frequent partial water changes to keep ammonia and nitrite below 0.25 ppm, since both are harmful to fish even at low concentrations. It works, but it stresses the fish and demands more effort from the keeper.

Seeding the filter with established media from a healthy tank dramatically speeds up either method. Squeezing out a dirty sponge filter or transferring ceramic rings from a running filter into the new one can cut cycling time to one to two weeks.

Bottled bacterial supplements can also accelerate the process, though results vary by brand. Regardless of method, always confirm the cycle is complete with a full test kit before stocking your tank.