Each aquarium has a biological cycle (usually named Nitrogen cycle) that depends entirely on the life hosted in the aquarium. The beneficial bacterial colonies that form in every aquarium are extremely important for the quality of the water and every break in the balance of this bacteria “shield” will bring a decline in the water quality and health problems for the fish and even deaths. Most if not all beginners in this hobby purchase their first aquarium and fill it with fish, almost always killing them, simply because they didn’t know anything about this important natural nitrogen cycle. I killed hundreds of fish before I learnt about this process. If you are reading this before getting your first aquarium, well… congratulations! You are a wise person and will enjoy your aquarium right from the start if you are patient and wait for the aquarium cycling to be complete.

I would offer a free brochure about this vital process in every pet shop or marketplace with every purchase of fish or aquariums.

The volume of water for the fish in nature is basically unlimited, so the waste products become diluted to very low concentrations that don’t harm them. Since the aquarium has a low volume of water compared to the natural environments, the waste produced by fish, excess food and dead plants quickly affect the quality of the water. Fortunately, the bacterial colonies convert these toxic substances into safe by-products. For newly setup aquariums, the water might be toxic for fish for a few days to weeks, before the bacterial biologic filter forms and is able to support the number of fish and waste produced. This is why it is always recommended to add only a few fish at a time, even after the cycle is completed, to allow the bacterial filter to adjust.

The Nitrogen Cycle – this is the process of converting the waste materials to safe by-products by the biologic filter. It starts in every aquarium, and it’s beginning is usually marked by a “milky” cloudy water for a few days. You must not add any new fish until the cycle is completed.

There are ways to start and complete the nitrogen cycle in your aquarium without fish, but I am sure you are looking forward to seeing fish in there, so we’ll discuss about the cycle WITH fish.

The Nitrogen cycle has 3 stages:

STAGE 1: Add only a few hardy fish to get this process started. A pair of corydoras and a pair of danio zebras would be just fine. The nitrogen cycle begins when live animals(fish, snails, etc) are introduced to the aquarium. The cycle requires live animals to go on to the next stages because the bacteria survive on these forms of waste described above. You don’t need to buy bacterial products to speed things up. Just wait for the cycle to complete, let the nature take its course. Some people suggest to “seed” the filter with bacteria from an older tank but opinions here vary – such bacteria exist everywhere so it’s enough to trigger the start of the cycle by adding a few fish and their waste products will help the bacteria multiply. The waste is converted into ammonia. The ammonia can be ionized (Ammonium = NH4) or unionized (Ammonia = NH3). Ammonia is very toxic to fish, so you want to have this down to 0. Ammonia rises in a few days after adding the fish, because of the waste they produce that is more than the biological filter is able to process. Feed the fish very light, once or twice a day (please remember, more food = more ammonia!).

STAGE 2: The Nitrosomonas bacteria eliminate the ammonia in a few days by converting it to nitrite (NO2) which is also toxic to fish.

STAGE 3: The Nitrobacter bacteria convert the nitrites into nitrates (NO3), which have a lower toxicity for fish if they are in moderate levels. Regular water changes keep these nitrates in safe ranges, but you might want to get nitrate tests to measure the levels from time to time, to make sure your fish are safe. Plants also use some of these nitrates, so a tank with many healthy plants is safer for the fish. Plants utilize nitrates as a fertilizer and everybody is happy!

The cycle is completed when nitrates are being produced (and ammonia and nitrite levels are zero). Usually this takes from 2 to 5 weeks, depending on the temperature (the higher it is, the faster the cycle ends).

REMEMBER: ammonia and nitrites should be 0 before you can add more fish. Most beginners don’t read about this extremely important cycle and keep adding fish while the ones they bought last week died “with no reason”. This is the “New Tank Syndrome”. Unfortunately, people kill thousands of fish before understanding this NATURAL cycle that can’t be avoided. Adding more fish before the cycle is done increase the ammonia levels so much that they are lethal, because the biological filter (bacteria) can’t process that amount. The same happens when you add too many fish even after a complete cycle, in a “mature” aquarium. So, it is recommended to add only a few fish from time to time.

If you already got a bunch of fish for your new aquarium, you can still fix the mistake and avoid fish death by buying ammonia neutralizing products.