Don't get wrong.

Cycled, cycled, cycled…I hear this word so many times, Everyday, as if it was the most important. The aquarium is already cycled, I am going to buy my first fishes… and after two weeks they are dead, or ill, or cornered behind a rock without being seen. That is because they were not right, and I'm going to complain to the store where I bought it: ” it is almost a month since I installed the aquarium, and it is already cycled because nitrite test is reading almost zero, but I've bought these fishes and they have died in two days…” and the same goes for corals and other invertebrates. I hear this so often. And I do not blame anyone, but there is much disinformation, and which is even worse: too much topic.

The problem arises when we think that once the aquarium is cycled we can begin to buy animals. In the best cases, we stop and think about the natural diet of the animal, if we will be able to provide it on a regular basis. Of course, an inner voice tells us: I am going to feed them with a little bit of frozen brine shrimp every day, or mysis, or lobster eggs…anyway, as I bought an Ultraskim XS skimmer. And so the fish won't go hungry and the water will keep clean, and in addition, I feed the snails with the leftovers, and they will be so happy that they will not stop…I won't go on.

Things are not so easy in the real world. The success of an aquarium is forged with time and patience and if you think that you can take shortcuts…well, everyone does what he prefers with his money. And there is also the option to set up an aquarium with clowns and damsel fishes, which are always nice to contemplate…

Cycling and maturation.

You can define cycling as the establishment of the nitrogen cycle. That is to say, the phase that every aquarium has to pass until, in a more or less stable way, a mechanism whereby organic stuff derived from its inhabitants' metabolism and uneaten food turns into ammonium is established (NH4, very toxic) nitrite (NO2, toxic) and subsequently nitrate (NO3, less toxic).

Ciclo del nitrógeno

As it is shown in the left picture, aerobic digestion carried out by nitrifying bacteria turns ammonia into nitrite and this nitrite into nitrate, therefore decreasing the toxicity of waste substances. From the 4th/5th week water would not be toxic for fish and the cycling stage is considered concluded.

And does that mean that we can now buy fish?

The answer has its own nuances: as a general rule, no. The fact that bacterial colonies capable of removing toxic nitrogen compounds are established in the system is just the first step, but there is still a little way to go. The aquarium is unstable, it has barely no life beyond the bacteria and some invertebrate, seaweed and sponges on the living rock. As soon as you enter any fish or food, a small peak of ammonium will show up, and it will soon be removed by the few bacteria that populate the system, but this will be a very slow digestion and for this reason the first fish introductions have to be very progressive and distant. It is recommended to start up by introducing a single fish, and a few days after one more, and so on until there is a stable nitrogen cycle.

But this is not all. Fish do not behave like mammals at lunchtime, fish do not have schedules. Their digestive system is simple and they need to eat continuously to thrive. From the surgeon fish that have to eat vegetal matter continuously, to the mandarin fish, and seahorses with their tiny mouth and stomach must eat small invertebrates incessantly. There are few exceptions, coinciding with predators: groupers, eels, scorpion fishes…that can have a meal from time to time.

There is another term, less used but equally important if not more: maturation.

An aquarium is considered mature when:

– The nitrogen cycle is strongly established. It is capable of converting all ammonium into nitrite and this into nitrate quickly and safely, even if we introduce many fish at once or if we feed them excessively, there will be a few bacterial colonies in sufficient quantity to neutralize these toxic compounds.

– There are bacterial colonies capable of denitrifying part of accumulated nitrates. These are bacteria that work in an opposite way: they tend to be located in anoxic areas (rock crevices, under the substrate…) and convert nitrate to gaseous nitrogen and oxygen which are released into the atmosphere. Normally in these anaerobic digestion only a small portion of the nitrate is removed, and therefore it accumulates; they require other molecules based on phosphorus and carbon, which tend to be limiting.

– It has developed a community of small invertebrates, algae and sponges that will sustain the first step of the trophic system. This will allow that most animals can find food naturally. This will strengthen their immune system and health

– The water is “aged”, it is less synthetic, much more natural. This water is less aggressive for any organism and is one of the reasons for:

  • Sponges do not grow well in less than six months old tanks (or somewhat less if the flow of nutrients is high).
  • Anemones in immature aquariums survival rate is very low. The recommendation is not to keep an anemone in less than six months old aquariums.

It seems clear that cycling and maturation are complementary but different concepts.

Matter of time?

Might say partially yes. In fact I don't know any aquarium that has matured in less than three or four months but I know many aquariums with more than one year that have not reached the maturation stage.

It depends on many factors: the diet, the type and quality of rock or substrate employed, whether or not bacteria are inoculated and type, the photoperiod, lighting, etc…I have noticed that low light aquariums, even dark systems tend to mature more quickly than the strongly illuminated aquariums. I can also state that aquariums with high flow of nutrients (which it is not the same as having high nutrient levels, we will talk about that topic later) mature really quickly, and also that there are commercial products that actually accelerate the maturation.


How to know if an aquarium is mature?

I use three criteria, If the three conditions are met, I consider that the aquarium is mature:

– Time. There is no maximum but minimum, It is not possible for an aquarium to be a mature system in less than eight or ten weeks. To put it in other words, I would never buy an achilles tang for a less than two months old system even if it had the most beautiful acropora collection you could imagine.

– Stability. It is important that the main parameters remain constant during a time, a mature aquarium is an system where nitrates are not fluctuating soon after you throw some food or add a fish.

– Micro fauna. A mature aquarium should have a substrate densely populated by copepods, amphipods, worms, brittle stars..even small algae and sponges.


That being said, the next time you think about buying a fish or invertebrate, maybe you should ask yourself if the aquarium is sufficiently mature and thus make a responsible decision.