Fishless Cycling

A method to establish the biofilter by dosing ammonia instead of using fish, with testing milestones and ways to speed it up by seeding media.

What it is

Fishless cycling establishes the nitrifying bacteria colony by providing an ammonia source without exposing live fish to toxic ammonia and nitrite. An external ammonia source feeds the developing nitrifiers until the biofilter can process a normal waste load.

Dosing ammonia

Pure ammonia (without surfactants or fragrance) is added daily without water changes, because water changes would remove the bacterial food source. A common starting point is roughly 1-2 ppm of total ammonia per day. Adding more than about 2 ppm per day, especially with a small filter or poor media, can make cycling unnecessarily long.

What happens over time

First, ammonia-oxidising bacteria reduce ammonia and nitrite begins to appear. Then nitrite-oxidising bacteria establish and nitrite falls while nitrate rises. The cycle progresses from an ammonia spike, through a nitrite spike, to a stable state where both read near zero.

When the tank is cycled

A tank is considered cycled when a dose of ammonia is reduced to about 0.25 ppm or less within 24 hours and nitrite has fallen to zero. At that point nitrate is present, and the biofilter can handle ongoing waste.

Testing

A liquid test kit for ammonia, nitrite, and nitrate is used to track progress, typically testing daily or every few days. Watching the ammonia drop, the nitrite spike and clear, and nitrate accumulate confirms each stage.

Speeding it up with seeding

Adding established filter media, substrate, or a bottled nitrifier product from a mature aquarium introduces ready-made bacteria. Seeding can shorten cycling to roughly 6-12 days, versus several weeks for an unseeded tank, and in difficult cases an unseeded ammonia-only cycle can take well over a month.

Conditions that help

Nitrifying bacteria grow fastest in warm water and near-neutral to slightly alkaline pH (around 7.4-7.8 is favourable), with good oxygenation. Excessively high ammonia, very low pH, or chlorine in untreated tap water slow or stall the process.