Ammonia - the product of fish waste, decomposition, and just general ammonification. Toxic to both freshwater and marine fish.
Cycling - everyone defines this differently, but I define it as 'to establish enough nitrifiers in an aquarium to fully handle ammonia produced by a full bioload'.
Microorganism - small living things. Include bacteria, but also other living beings, such as algae, fungi, and so on. There is some debate amongst scientists about what can exactly be classified as a microorganism. Mostly irrelevant to the aquarium-keeping hobby, so don't worry too much about it.
Nitrification - The process in which ammonia is oxidized to nitrite then nitrate. Do not worry about what 'oxidized' means, you can use the term 'converted' instead. Not exactly important, but may be of interest: ammonia can actually be directly converted to nitrate, by microorganisms such as Nitrospira inopinata.
Nitrifier - A microorganism that either converts ammonia to nitrite, or nitrite to nitrate (or ammonia to nitrate directly, see nitrification). Common genera ('groups') of nitrifiers are Nitrosomonas, Nitrosospira, and Nitrospira. In marine environments, certain nitrifying Thermoproteota species are also common, specifically from the genera Nitrosopumilus and Nitrososphaera.
Nitrate - The final product of nitrification. Nitrate can be consumed by certain organisms, including microorganisms in anaerobic processes, or by plants and algae. Nitrate toxicity varies greatly depending on live stock species, but generally not as much as ammonia or nitrite.
Nitrite - The product of ammonia oxidation, which is then oxidized to nitrate. Nitrite is very toxic to freshwater fish, but needs to be at very high concentrations to be toxic to marine fish. Toxicity varies between marine fish species, but probably around 100+ppm is enough to be lethal for some species, and higher for others. I would suggest nonetheless keeping nitrite below 25ppm for marine systems, as even if not lethal, higher concentrations of nitrite can still cause disease.
Today, I want to answer a very important consideration that has caused a lot of debate in the hobby for a very long time - what do nitrifiers feed on?
Nitrifiers have generally been described as obligate autotrophs by scientists, specifically the species that tend to be found in our filters and bottled bacteria products.
What does this mean? Well, obligate autotrophs are living beings that must generate their own 'food' from carbon dioxide and other readily available inorganic matter, rather than organic matter.
There are nitrifiers that are heterotrophic instead, or in other words consume 'food' from the environment for growth. Think of these like animals versus autotrophs that are plants. However, the nitrification process that these nitrifiers catalyze is a bit different from what we want to achieve in our aquariums - a process that can be driven by just the presence of pure ammonia, and that is not dependent on the consistent growth/reproduction of nitrifiers. The reason is because, autotrophic nitrifiers can remain persistently consistent in population and continuously consume ammonia day by day, which of course makes for very effective filters.
So if you are thinking that - hey, then does that mean one does not need to add fish food, shrimp from the deli, or something similar to feed these nitrifiers during the cycle? - then yes, you are absolutely correct! The nitrifiers actually cannot 'consume' the food you provide. They have to produce their own from carbon dioxide and water. Note, 'consume' here is in quotation marks, because they may be able to consume byproducts of the degradation of that food by other microorganisms. Ammonia of course, is one of these byproducts.
Now, I want to clarify - like all living beings, nitrifiers need more than just carbon for growth. They need nitrogen and phosphorous as major elements, along with many other elements. However, these are pretty much readily available in the environment, including from your tap water.
This is why I have put 'food' in quotation marks above. Because in such cases, I was referring specifically to 'food' as a carbon source. But yes, the food you feed can provision the nitrifiers with nitrogen, phosphorous, and other elements they need. Theoretically, that is. In practice, nitrifiers grow far more slowly than other microorganisms. So likely the food will be consumed not by the autotrophic nitrifiers, but other microorganisms, specifically heterotrophic bacteria, that you do not care about during the cycle.
So you may be thinking - then, if I actually go towards the opposite extreme and try to avoid adding in food to my aquarium, would that promote the growth of nitrifiers? Yes. Absolutely. By limiting the carbon source in an aquarium, you are preventing the growth of heterotrophs that has to consume the food. Instead, the nitrogen and phosphorous, along with minor elements readily available in water, will be all that is needed for the nitrifiers to grow.
Yes. That is why ammonia-dosing for fishless cycling is preferable.
Yes. That is also why you may want to use tap water, rather than water that is too 'filtered' like RODI water. While one can easily supplement nitrogen in the form of ammonia-dosing, other chemicals may not be as readily available.
Tada! ^_^ That is the answer.
Questions? Ask in the comments section or in the forum.