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what do people mean when they say that they have fish for cycling the aquarium? What is it for?

If that FAQ didn't sum it up well enough for you, people will put very hardy fish that produce a lot of waste, like a goldfish or some zebra danios, into an aquarium to cycle it. The waste they leave is a form of ammonia. That ammonia is the key ingredient in cycling an aquarium. The cycle is the process of establishing beneficial bacteria, which is naturally in the atmosphere, into an aquarium. That bacteria changes the ammonia into nitrite. A less toxic product. Then different bacteria change that nitrite into an even less harmful state, nitrate. That nitrate is still harmful, but not until it gets to around 40 ppm (parts per million). This nitrate should be removed through water changes performed by the aquarist once a week.