What’s “the Cycle” and why is it so important?

Everyone has heard the old story, “I filled my aquarium with fish, and they all died,” usually followed by “so now I don’t keep fish anymore.” There are plenty of ways to kill all of your fish, but not understanding the cycle is one of the most common ways I’ve come across.

The nitrogen cycle refers to the process of turning harmful fish waste (Ammonia), into a much safer end product, called nitrate. Most people think of their aquarium as just a box, but it is literally a living, breathing ecosystem that helps us keep our fish alive by utilizing waste eating bacteria. But it doesn’t start out that way. We need to develop it.

Any new tank is, just that: a sterile, glass tank. But after we add gravel, water, etc. and are ready for the fish, we choose the right amount of durable, hardy fish to add. Then the days go by, and we keep feeding them, and they keep producing ammonia. Every day. If you had an ammonia test kit, you could see the ammonia gradually rising every day to VERY toxic levels, but eventually, the ammonia level begins to drop, and it becomes zero and should stay there every day after. This means that your beneficial bacteria have produced enough of themselves to turn your daily ammonia into something else: nitrite.

Nitrite is also very toxic, and not to be confused with the much safer end product, nitrate. An easy way to remember the difference is to use the old saying “the difference it night (nitrite) and day (nitrate)”, since the vowel sounds are the same, and they are produced in that order. It is important to remember this difference because it can mean not killing your fish in the future.

But as ammonia levels fall and nitrite begins to be produced, daily water tests would show the nitrite also rise to deadly levels, drop back down to zero, and stay there. So by this time, a water test would show no ammonia, no nitrite, and all of that waste that has been getting processed this whole time would show up in a nitrate test.

Now although this end product is safer, we can compare it to smog. The smog in the air varies from city to city, but the more smog there is, the more it may affect our health. Nitrate works the same way. This is the main reason we change the water in our aquariums, to dilute out the waste.

But once the cycle is complete (no ammonia or nitrite) one can begin adding more fish to their aquarium. But because we only have enough bacteria to break down the waste of the current fish load, new fish should be added in small numbers and over time, so the bacteria can have a chance to grow and rise to the task. Adding too many fish or adding fish while the tank is cycling can mean death for the new fish, or even all of them!

But knowing this process can not only help you to start an aquarium successfully (with no fish losses!), but it is foundational to understanding the way an aquarium actually functions. Much like anything, the more you understand about something, the easier it becomes, and this is especially true for aquariums, in every area. From cleaning your tank, to buying fish, to keeping a thriving reef aquarium, a little bit of knowledge will go a long way particularly the cycle.

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