Eli5: What does it mean to ‘cycle an aquarium’?
It literally just refers to removing all of the water and replacing it with clean filtered water.
This needs to be done fairly regularly because there are a lot of toxic chemicals that can build-up in water and not be obvious until they start killing the life in the aquarium. Ammonia is a pretty big and important example, and it is always building up because all animals secrete it in their urine.
‘Cycling an aquarium’ refers to growing a bacterial colony that carry out a nitrogen cycle.
Fish or other animals will produce ammonia as one of their wastes. It’s toxic, but bacteria can break it down into nitrite. Nitrite is still toxic, but another set of bacteria can break it down into nitrate. Nitrate is far less toxic, and can even be used by plants and algae.
Cycling an aquarium involves growing those bacterial colonies so that they’re large enough to process all the waste that the inhabitants of the aquarium produce into at least nitrate. A cycled aquarium is simply one that already has all the bacteria necessary to do so.
It means – fish eat, then they poop.
Fish poop is toxic to fish if it builds up and isn’t processed, so it needs to be dealt with. Some bacteria are good at dealing with poop and converting it to substances that are harmless to the fish.
It takes time for this kind of bacteria to grow so that there are enough of them to deal with the poop of a whole tank of fish.
Cycling a tank means you’re taking a new clean system with no bacteria in it, and introducing some kind of food that good bacteria wants (uneaten fish food, fish poop etc). This waste is eaten by the good bacteria (that exist everywhere in tiny amounts) the tank needs. As the good bacteria process the food and multiply, they can handle more waste. This increase in good bacteria is “colonising” the tank. When you’ve cycled your tank, it means you’ve given the good bacteria enough time to grow and multiply to a point where they can reliably process the waste in the tank to keep the environment stable and healthy for fish.
It means that you’re getting your tank ready to support aquatic life by building up the nitrogen cycle within your tank. TL;DR version is that the nitrogen cycle processes the waste in your tank.
We can keep fish in boxes thanks to a variety of bacteria species that process waste products from your critters and turn them into something less toxic. The most toxic chemical (barring environmental toxins, like copper or other heavy metals) that will be produced in your tank is ammonia; even in relatively small quantities, free ammonia can kill everything in your tank.
Thankfully, life has been around the block a few times and there are bacterial species that can eat ammonia. After eating ammonia, they then excrete a chemical called nitrite… which is still toxic to animals, but less so than ammonia.
Again, though, there are bacterial species that eat nitrite. So they’ll take it up and excrete nitrate. So there’s your (partial) cycle: Fish eat food and produce fish poop, which releases ammonia. Bacteria eat ammonia and excrete nitrite. Different bacteria eat nitrite and excrete nitrate. And if you have a typical aquarium, this is where the cycle ends. Incidentally, nitrate is, once again, still toxic to most animals, but much less so than nitrite or ammonia. So unless you have specific conditions in your tank, you will need to regularly change the water in your tank to flush the excess nitrate out of your system.
It is possible to complete the cycle in an aquarium, but it’s more difficult to create these conditions. To do so, you need something that converts nitrate into nitrogen + oxygen. This can be done by bacteria, but only in an environment that is very low in oxygen. This requires either highly porous live rock or a deep (4″-6″) sand bed. However, if you know anything about gardening, you know that we have another word for nitrate: Fertilizer.
And that suggests the other way to complete the cycle within your tank: Introduce plants or other photosynthetic organisms. So if you have a well-balanced planted tank (or a well-balanced reef tank, with corals, tridacnid clams and macroalgae taking the place of plants), you can complete the nitrogen cycle within your tank without every doing water changes. Granted, you might want to do water changes for other reasons, but as far as the cycle is concerned you can process 100% of your fish waste within your tank.
And of course, if you don’t have any plants or other organisms, nuisance algae will happily consume that nitrate for you. But there’s a reason that we call them nuisance algae… but that’s a different subject.
Fish poop out ammonia, which can very quickly build up to toxic levels in an aquarium. When a tank has been cycled it hosts a colony of nitrogen-fixing bacteria in its filter and substrate. The bacteria oxidize the ammonia into nitrate and then nitrite, which is harmless. “Cycling the aquarium” means establishing the bacterial colony on a new tank, typically by transferring filter media from an older aquarium.