Eli5: What does it mean to ‘cycle an aquarium’?
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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.
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