Eli5: What does it mean to ‘cycle an aquarium’?
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There’s a ton of things that can harm fish, like dish soap and even their own waste(poop, uneaten food). But in a ‘cycled’ aquarium, the waste is able to get processed in a way so that it’s not harmful anymore.
It all has to do with the nitrogen cycle. The waste produces ammonia(very harmful), which turns into nitrate(less harmful) after it gets eaten by ammonia-eating bacteria, then nitrite(not harmful) after being eaten by nitrate-eating bacteria. One set of of going from ammonia to nitrate to nitrite is a complete cycle.
But when a tank is newly set up, the ammonia just sits in there causing harm because the tank doesn’t yet have the resources(ammonia-eating bacteria) to change it into nitrate. It has to grow some over time, then it has to grow the nitrate-eating bacteria, too. The aquarium needs to ‘cycle through’ these steps multiple times in order to grow enough bacteria to handle fish, hence why its called ‘cycling’ the aquarium. A cycled aquarium continues this cycle for the life of the tank(otherwise, major problems).
There are two main methods of cycling an aquarium: without fish, or with fish.
Cycling with fish is harder and not recommended for beginners because without that bacteria set up, the toxic ammonia can harm or kill the fish/other aquatic life in the tank.
Although for the experienced, its doable with a combination of strong, more ammonia-resistant fish(hardy), lots of constant measuring of each of the 3 levels(ammonia, nitrate, nitrite), and water changes to lower the levels down to a degree that won’t do too much harm.
However, this can easily lead to dead fish and an uncycled aquarium or improperly cycled aquarium is the most common and easiest way beginners kill off all their fish.
Cycling without fish, or a fish-less cycle, is much more recommended and healthier for your fish, because they won’t be around for the toxic part.
The first step is to set up the aquarium as if the fish were already in there with the water, gravel, decorations, filter, heater, air stone, etc. and let the filter and everything run.
However do not add the fish yet. Instead you can add a bit of fish food to your tank (or you can use some other way to get ammonia, like a frozen shrimp or straight-up ammonia.) The ammonia-eating bacteria will then start to grow on its own just with the food source.
Eventually(usually couple weeks or so), the ammonia-eating bacteria comes and produces some nitrate, then eventually(another couple of weeks), the nitrate-eating bacteria comes and produces some nitrite(the good stuff).
You need a testing kit to test the tank periodically to see if you have just ammonia, or if you made it to the nitrate or nitrite stage. You may also need to add more ammonia.
After all that, the tank has completed one cycle, but is still not ready for fish yet(close!). The ammonia-eating bacteria and nitrate-eating bacteria have to keep growing, keep producing the better stuff, until you have enough to handle all the fish you want to put in the tank.
You can test if you have enough bacteria by adding more ammonia and testing how long it takes to get to the harmless nitrite. If you add enough ammonia that your fish will produce(their bioload), that turns into nitrate then nitrite quick enough not to cause harm, you are finally ready for the fish. (:
Here are some extra tips I learned along the way:
Goldfish are messy and have a particularly high bioload compared to other fish of the same size/tank requirements(and should not be kept in a bowl-they like their surface water).
Snails and other ‘clean-up crew’ creatures can eat fish waste(if you get the right type) but have their own bioload and add to the total waste of the tank(bioload), not subtract.
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.
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.
‘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 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.
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.
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