Cycling a New Aquaponics System Before Adding Fish

A brand new aquaponics system looks ready to go the moment the pump starts running and the water begins to circulate. It is not. The gravel beds, pipes, and tank walls are biologically bare. There are no bacteria in useful numbers to process fish waste, and without those bacteria, ammonia will build up quickly enough to kill fish. Cycling is the process of growing those bacterial colonies before, or very carefully alongside, the fish you intend to keep. It takes weeks, not days, and patience during this period pays off for the entire life of the system.

What the nitrogen cycle actually does

Fish excrete ammonia directly through their gills and in their urine. Decomposing food and other organic matter in the tank also produces ammonia. In a lake or ocean the volume of water dilutes this to safe levels, but in a closed system it accumulates rapidly and becomes toxic. As described in the IBC of Aquaponics, excessive ammonia causes extensive tissue damage (especially to the gills and kidneys), impaired growth, reduced resistance to disease, and eventually death.

Two groups of bacteria do the work of making that ammonia safe. Nitrosomonas sp. consumes ammonia and converts it to nitrite. Nitrite is considerably less toxic than ammonia, but it is still dangerous because it prevents fish from taking up oxygen properly. A second bacterium, Nitrobacter sp., then consumes nitrite and converts it to nitrate. Nitrate is far better tolerated by fish and is the primary nitrogen nutrient that plants absorb. This three-step process, ammonia to nitrite to nitrate, is the nitrogen cycle. A system is said to have cycled when it has enough of both bacterial populations to process ammonia and nitrite completely and continuously.

The bacteria do not convert these compounds because they have a preference for a particular form of nitrogen. As explained in the Aquaponic Solutions Water Chemistry fact sheet, they do it because the chemical exchange releases energy, which they use to run their own metabolism. Cleaner water for fish and plants is a fortunate by-product of the bacteria meeting their own energy needs.

Why cycling takes so long

Nitrifying bacteria reproduce slowly. According to BYAP Magazine (Issue 1, by Steve Cacchione), under optimal conditions Nitrosomonas sp. may double every seven hours and Nitrobacter sp. every thirteen hours. In realistic system conditions, both will double every fifteen to twenty hours. By comparison, a single E. coli bacterium would produce a population exceeding 35 trillion cells in the time it takes a single Nitrosomonas sp. cell to double once. There is no shortcut that changes this biology.

The two populations also have to build in sequence. Nitrobacter sp. will not reproduce in useful numbers until Nitrosomonas sp. is well established and producing plenty of nitrite for it to consume. This is why, as described in the IBC of Aquaponics, you will see an ammonia spike early in cycling followed later by a nitrite spike. Each spike represents bacteria catching up to their food source. As a general rule, BYAP Magazine notes that a new system requires about four weeks to cycle at around 20°C, and longer in colder water.

What the bacteria need to thrive

Both Nitrosomonas sp. and Nitrobacter sp. need surfaces to colonise. They will grow on gravel, sand, expanded clay, synthetic biomedia, and the walls of tanks and pipes, but they require a physical substrate for optimum growth. They also need dissolved oxygen in the water to live and work, so good aeration matters from the start.

Temperature has a significant effect on how quickly the colony builds. BYAP Magazine reports that the optimum growth temperature for nitrifying bacteria is between 25°C and 30°C. Growth rate drops by 50% at 18°C, by 75% between 8°C and 15.5°C, and stops entirely below 4°C. The bacteria die at 0°C and also at sustained temperatures above 49°C. In cold water systems, Nitrobacter sp. is less tolerant of low temperatures than Nitrosomonas sp., which means nitrite can accumulate dangerously during cold periods even in an otherwise established system.

pH matters too. The IBC of Aquaponics notes that the optimum pH range for Nitrosomonas sp. is 7.8 to 8.0, and for Nitrobacter sp. is 7.3 to 7.5. Below pH 7.0, Nitrosomonas sp. growth slows. It is inhibited at pH 6.5, and all nitrification stops at pH 6.0 or below. Many practitioners aim for pH 7.0 to 7.2 as a working compromise that suits fish, plants, and bacteria. BYAP Magazine suggests that adding calcium carbonate (shell grit, eggshells, or calcium carbonate powder) is a straightforward way to hold pH steady around 7.4 during cycling, because calcium carbonate stops dissolving at that level and provides a natural buffer until it is depleted.

Providing an ammonia source

Without ammonia, Nitrosomonas sp. will not establish in useful numbers, and without nitrite, Nitrobacter sp. will not follow. You need to introduce an ammonia source from the beginning of cycling. The IBC of Aquaponics describes several approaches, each with practical trade-offs.

• Fishless ammonia dosing. Adding pure food-grade ammonia, urea fertiliser, or aged urine to the tank gives you direct control over ammonia levels without putting any fish at risk. Practical Aquaponics for Everyone by Gary Donaldson recommends this approach precisely because it avoids stressing fish during the vulnerable cycling period. Urea and household ammonia both work, but the IBC of Aquaponics cautions that care is needed with dosing and that household ammonia must be food-grade only, free of perfumes and other additives.
• Decomposing organic matter. A dead prawn, some rotting fish, or a daily dose of fish feed added to the empty tank will release ammonia as it breaks down. This is a lower-tech option that requires less precision but can be harder to control.
• Feeder fish or fingerlings. The IBC of Aquaponics describes this as possibly the simplest method and the one they recommend for new systems. Cheap bronze comets or goldfish can be used as cycling fish before introducing your final species, or you can stock fingerlings of your intended species directly. Either way, the IBC of Aquaponics is firm: feeding must be kept to a strict minimum for the first two months, no more than one tablespoon of feed per day per 500 litres of media. Overfeeding produces excess ammonia that the still-developing bacteria cannot process. BYAP Magazine makes the same point: if too many fish are used during cycling, ammonia and nitrite can reach levels that kill or permanently damage them.
• Seeding from an existing system. The IBC of Aquaponics notes that if you can obtain water, gravel, or filter media from a healthy established aquaponics system, aquarium, or pond, adding it to your new system gives you a head start. The bacteria are already present in useful strains and the colony will establish faster than it would from scratch.

What to watch for as the cycle progresses

Test your water daily during cycling. A kit that measures pH, ammonia, nitrite, and nitrate will show you exactly where you are in the process. Both the IBC of Aquaponics and Practical Aquaponics for Everyone recommend this level of monitoring.

The sequence you should see is predictable. Ammonia rises first, sometimes to high levels, as fish or your added ammonia source produces more than the small initial bacterial population can process. Then, as Nitrosomonas sp. establishes, ammonia begins to fall and nitrite starts to climb. This nitrite spike is a critical period: Nitrobacter sp. has not yet built up to match the nitrite load, and as the barrelponics manual notes, nitrite is very toxic to fish at this stage. Finally, as Nitrobacter sp. catches up, nitrite falls and nitrate rises steadily. Rising nitrate with falling ammonia and nitrite is the sign that cycling is complete.

Practical Aquaponics for Everyone describes the system as cycling when ammonia and nitrite readings have each reached at least 5 ppm and then returned to zero. From that point, detectable ammonia is a warning sign of a problem rather than a normal condition.

During the process, perform partial water changes if ammonia or nitrite climbs to levels likely to harm fish. The barrelponics manual offers a simple principle: dilution is the most immediate response to dangerously high readings. Remove some of the water and replace it with dechlorinated or aged water.

What going wrong looks like

The most common mistake is impatience. The barrelponics manual describes losing fish by adding more stock at exactly the wrong moment: ammonia had dropped (suggesting the system had cycled) but nitrite had not yet been brought under control because Nitrobacter sp. had not established. Ammonia reaching zero is not the end of cycling; it is roughly the halfway point.

Overfeeding is the second most common problem. Fish produce ammonia even when not fed, just at a lower rate. Adding extra feed on top of that can push ammonia beyond what the developing bacteria can handle. Keep feed minimal until you have confirmed continuous nitrate production and stable, near-zero ammonia and nitrite readings.

A pH that drifts too low will slow or stop the cycle entirely. If pH falls below 7.0 and ammonia starts climbing again in an otherwise established system, suspect that nitrification has slowed due to pH rather than a bacterial die-off. Add a calcium carbonate buffer and monitor closely.

In cold climates or in winter, the cycle will simply take longer. Doubling times stretch out as temperature drops. There is no way to rush this other than warming the water. In the wet tropics and other warm climates, a summer cycle at 28°C will establish far faster than a winter cycle at 15°C in a temperate system.

Once the system has cycled

A cycled system has a living bacterial community spread across the surfaces of your grow media, tank walls, and pipework. This community is more resilient than the early-stage colony: as BYAP Magazine notes, once a system has a full complement of micro flora and fauna at work, there is an inherent synergy that allows wider environmental ranges to be tolerated than lab studies of individual strains would predict.

Add your fish stock gradually. Stocking the full intended load immediately after cycling puts the bacteria under sudden pressure from a large ammonia increase. Give the bacterial population time to grow in response to each increase in fish load. Continue testing ammonia and nitrite regularly, especially after adding new fish, and treat any ammonia reading above zero as something to investigate and address quickly.

The weeks you spend cycling are not wasted time. They are the foundation that everything else in the system depends on.