One of the most important, and least understood aspects of successful aquarium keeping is biological filtration and its function in the nitrogen cycle. Aquarists, often unfamiliar with this process, become disillusioned at the high death rates of their new aquatic friends after setting up a new aquarium. It can be very painful, especially for people keeping an aquarium for the first time, and they end up feeling responsible for the deaths, and frustrated too, since they can’t see anything wrong in the set-up, having already accounted for and treating Chlorine in the tap water.

As much as 60% of the fish sold for a new aquarium die within the first month. As a result of this, 2 out of every 3 new aquarists abandon the hobby within the first year itself.

This debacle is so well known that aquarists even have a term for this, the ‘New Tank Syndrome’. The only solution is to ‘cycle’ the new tank.

To understand what and why, let’s have a look at the Nitrogen Cycle..

Ammonia is produced from decomposition of fish waste, excess food, and animal and plant tissues. Additional ammonia is excreted directly into the water by the fish themselves. Ammonia (NH3) is highly toxic to fish.

The effects of ammonia poisoning in fish are well documented and include extensive damage to tissues, especially the gills and kidney; physiological imbalances; impaired growth; decreased resistance to disease, and in many instances death. Stress related to Ammonia often cause the fish to stop eating, and the fish wither away over a period of time.

What level of Ammonia is safe? Unfortunately, the answer is 0 ppm (or mg/L).

Ammonia is broken down into Nitrites (NO2-) by a certain kind of bacteria, called Nitrosomonas bacteria. Unfortunately Nitrite is almost equally toxic for the fish. Nitrite poisoning inhibits the uptake of oxygen by red blood cells. Known as brown blood disease, or Methemoglobinemia, the hemoglobin in red blood cells is converted to methemoglobin.

In this case, the haemoglobin in the fish’s blood is not able to carry oxygen from the gills to the heart, even though the water may be well aerated and dissolved oxygen levels healthy. This problem is much more severe in fresh water fish than in marine organisms.

What levels of Nitrites are safe? Again, the answer is 0 ppm (or mg/L).

Once Nitrites spike occurs, as second class of bacteria, called Nitrobacter, come into play, and they break down the Nitrites to Nitrates (NO3-).

Are Nitrates safe? Sadly, no again.

But nitrates are manageable, as they are toxic to fish in much higher concentrations than Nitrites or Ammonia. Nitrate concentrations upto 40 ppm are manageable by most fish. Partial water changes done on a regular basis is the answer to the problem of Nitrates. How much to change, and how frequently this change is required, depends on the tank size, setup and the bio load (read number and type of fish) in the tank.

The successful aquarist realizes the importance of establishing the nitrogen cycle quickly and with minimal stress on the aquarium’s inhabitants. Ideally a tank should be cycled before introducing the fish, but if you are like most people including me, you would have already bought the fish with the aquarium.

So how do we cycle a tank with the fish?

Complicated, requires effort, stressful for the fish, but doable… How, did you ask, read up on the section on Cycling the New Tank.