What Is Aquarium Cycling

or Why Are All My New Aquarium Fish Dying?

Many people buy an aquarium, set it up with the filters, decoration, and heating, and after a few days they start adding in fish. Those fish proceed then to die over the period of a few days, leaving the new aquarium owner understandably upset. If this sounds familiar, don’t worry. Every new aquarist, at some point or other, needs to learn about the nitrogen cycle, and why cycling your aquarium is a vital part of keeping healthy fish.

The Nitrogen Cycle In Aquariums

Nitrogen is a component of all organic matter, a vital element of proteins which are the building blocks of all living beings. Nitrogen is introduced into the aquarium when you feed the fish.

Fish waste, urine and uneaten food are soon broken down into either ionized (NH4) or unionized ammonia (NH3, or Ammonium). Ammonium, which is not toxic to the fish, is present if the water’s pH is below 7. If the pH is above 7 (most common situation in tropical aquariums) then ammonia will be present and this is highly toxic to fish. While any amount above 0 ppm (part per million) is dangerous, concentrations above 2ppm are quickly lethal to fish. Ammonia begins to rising about 3 days after introducing fish.

In Nature, Nitrosomonas bacteria oxidize the ammonia and create nitrite, which is also highly toxic to fish but much less than ammonia. Concentrations as low as 1mg/l can kill some fish, but it’s still an improvement over Ammonia.

In a freshly set up aquarium there are no Nitrosomas bacteria, and such the ammonia is not processed, leading to the death of the fish. However, by the end of the first week after introducing fish bacterial colonies usually have started establishing themselves on the filter and substrate of the aquarium, and Nitrite levels will start to rise. This process can be helped by using bioactive aquarium additives which provide seeding bacteria to quick start the process. Also, using substrate or filters from an old and established aquarium also helps.

The third step of the nitrogen cycle begins when Nitrobacter or Notrospira (in marine environments) bacteria convert nitrites into nitrates. Nitrates are even less toxic than nitrites, and fish can usually deal with low to moderate levels of them. In nature, Nitrates are then eliminated by plants, or just diluted on the environment. In an aquarium, routine partial water changes get rid of them.

So What Does Cycling An Aquarium Mean?

The process of cycling an aquarium consists on allowing the required bacteria to establish themselves to replicate the natural Nitrogen cycle, and it can take from several weeks to months. Also, fish tanks can enter into mini-cycles when too many fish are added and the existing bacteria colonies can’t cope with the Ammonia generated, but this is more common with small aquariums where a dead fish decomposing behind a rock can throw everything off balance. Eventually, most aquariums will cycle on their own however the loss of fish can be significant if the process is not controlled.

There are several ways to cycle an aquarium with minimal distress to fish. Among them, the most popular is Fishless Cycling, where a source of nitrogen (that is not a living fish) is introduced in the aquarium. You can also use hardy fish for cycling, but you will require very frequent water changes and a good water conditioner. And yet a third option is known as Silent Cycling, where you use plants to process the Ammonia and Nitrites. There is no hard way to estimate when an aquarium will start and finish cycling, so in order to cycle a fish tank you will need to invest in a good testing kit for Ammonia, Nitrites and Nitrates.

Cycling An Aquarium With Fish On It (Fish-in Cycling)

You may have already introduced fish in your aquarium and have no cycled aquarium to house them, or maybe your aquarium has entered into a mini-cycle. No problem, an aquarium can be successfully cycled with fish on it though in order to avoid fish deaths you will need to be really strict with testing the water and water changes. A good ammonia remover and water conditioner such as Seachem Prime also helps greatly.

We have found that aquarium biological enhancers (which are basically a cocktail of Nitrogen processing bacteria) also help, but there is no consensus about this among the fishkeeping community. Personally, I think they do no harm, and anything that can make my aquarium cycle quicker and safely for my fish is good in my eyes. Some people recommend feeding the fish extra so the uneaten food and excess fish waste help start the process sooner.

You will need to test the water for Ammonia and Nitrites daily, and perform 40% water changes or more every time Ammonia or Nitrite is present. Alternatively, you can just pour Prime at 5 times the recommended dosage in order to make the Ammonia into a non-harmful compound that is still available to the bacteria but won’t affect the fish. The only issue with dosing with Prime for ammonia control is that your ammonia meters will still give positive, as the chemical reaction used transforms Prime’d ammonia into regular one in your test vial. Eventually, one day there won’t be any ammonia. Or Nitrite. And your tank will have cycled.

This method works, but if you have very sensitive fish or fish that tolerate badly big changes of water you may lose several of them. Using Prime instead of water changes can help with survival, but ideally you would cycle an aquarium using either plants or no fish at all.

Cycling An Aquarium Without Fish (Fishless Cycle)

All the bacteria need in order to be happy and colonize your aquarium is ammonia. This means you don’t need to add fish to the aquarium as long as you have an alternative source of ammonia, which can be decomposing food (a prawn or fish food) or even pure ammonia that you can buy as a cleaning product in a supermarket. Regular testing is also necessary so you can identify when Ammonia rises (phase 1), Nitrite rises (phase 2) and finally when Nitrates rise (phase 3) and the nitrogen cycle is complete. Bacterial supplements for aquarium (quick start conditioners with live bacteria) can help speed things up.

Fishless cycling with ammonia usually takes about 4 weeks, and should be conducted in the dark. Otherwise you will find that your tank fills with algae, as algae growth is encouraged by abundance of usable nitrogen composts. If you intend to plant your aquarium heavily, consider doing a silent cycle instead. Otherwise when you add plants that compete for the ammonia your bacteria will starve and you may go through a mini-cycle again.

Cycling An Aquarium With Plants (Silent Cycle)

In a tank that is heavily planted (more than 50% of the aquarium for a large tank) you may see that the ammonia levels never rise significantly. This is because your plants are eating the ammonia before it can be tested, and adding it to their biomass. On the meanwhile, the ammonia processing bacteria starve and fail to colonize your tank. This doesn’t mean that your tank is not suitable for fish, but can lead to issues once you introduce a lot of fish in the tank and disturb the balance.

Plants are not affected by ammonia, in fact, the contrary happens. Plants are very good at eating ammonia, but sadly algae are very good at it too and your planted aquarium may soon acquire a disgusting green tinge. The best way to avoid this is either by via of CO2 injection, which is complex and can be expensive unless your goal is an underwater garden, or by introducing algae eating fish as soon as the plants are established.

In order to avoid algae, the best thing to do is regulate the amounts of light. Planting an aquarium with plants that don’t require a lot of light in order to do a silent cycle will help, as high light and ammonia is a foolproof recipe for algae. Start with planting your aquarium and add a small amount of ammonia to see whether the plants can cope with it. If they can, add a few fish, preferably some algae eating fish to keep the algae under control, and keep adding fish and measuring the water every few days to make sure the ammonia levels don’t rise. Permanent ammonia meters such as Seachem Ammonia Alert are great for this, as you can see changes in the level of ammonia almost in real time. If that happens, a water change should get rid of it.

Silent and Fishless cycling are considered more humane, as no fish are exposed to ammonia in the process. However, no matter which fish tank cycling method you use, testing for ammonia daily or weekly once the bacterial colonies are established and your aquarium has finished cycling is still necessary. An established tank can go back into a mini-cycle if the balance between the ammonia production levels and the bacteria levels is disturbed, such as when fish is added, too much food is added to the aquarium or a fish dies and the corpse is not removed from the aquarium quick enough and starts to decompose.