Cycling (nitrogen cycle)

Cycling is the process by which the colony of nitrifying bacteria is established in a new aquarium. Without complete cycling the fish die.

Detailed explanation

Every new aquarium starts out sterile. There are no bacteria. Put a fish and the ammonia it produces has no one to transform it. It accumulates and becomes toxic. Then the first Nitrosomonas bacteria arrive and convert ammonia into nitrites. Then the Nitrobacters convert the nitrites into nitrates. This process takes weeks.

Classic cycling works like this: add a source of ammonia (fish food, pure ammonia, a dead shrimp). Wait. Measure ammonia and nitrite every 2-3 days. When ammonia rises and then drops to zero the Nitrosomonas have established themselves. When the nitrites also rise and then fall to zero, the Nitrobacter are also ready. At that point you can insert the first fish.

The time depends on the temperature. A marine aquarium at 25 degrees cycles in 4-8 weeks. A tropical freshwater at 26 degrees in 3-6 weeks. A spring pond with water at 12 degrees can take 8-12 weeks because the bacteria work slowly in the cold.

Commercial bacterial products (Seachem Stability, Prodibio Biodigest) can speed up the process but do not eliminate it. However, patience is needed.

Live rock in marine aquariums carry an already active bacterial colony and can reduce cycling to 2-3 weeks. But they must be fresh, not dry, live rock.

The most common mistake made by beginners? Add fish on the first day. The second most common mistake? Never measure ammonia and nitrite during cycling and consider it finished after an arbitrary time.

Reference values by aquarium type

How it is measured

Monitor ammonia and nitrite every 2-3 days during the process. The cycling is complete when ammonia and nitrite are stable at zero for at least 3 days consecutively with the presence of nitrates.

What happens when it is too low

Not applicable. Cycling is an event, not a parameter.

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