That's a fairly broad question as there are a few different ypes, and many different brands that these different types are sold under.
Essentially there are two sorts of bacteria, Autotrophic, these are the bacteria that perform the bulk of nitrification from ammonia, to nitrite to nitrate, and heterotrophic bacteria, which can also performn removal of ammonia and nitrite, but not as well as autotrophs, but mostly assimlate solid watse, dissolved watse, and nitrate into nitrogen gas.
When starting up a new tank, Using Autotrophic bacteria, especially when cycling your live rock, will help to speed up the process, generally you onl;y need to add this once at the start, until ammonia and nitrite levels reach zero, and then again at the start of adding fish. It also helps at the stage when first adding LR to add a hetertrophic mix (waste assimilators) to help speed up the process.
Then usually, you add a heterotrophic bacteria mix (most often called pro-biotics) to help get effcient strains of bacteria into the tank to help control nutrient. This is usually on a weekely, or sometimes on daily basis, because these bacteria reproduce so quickly, they soon become far removed (mutated, like making a copy of a copy of copy over and over again) and are not as effcient as they once were, they also reproduce quickly and can eat themselves out of a food source.
Now there are a few schools of thought about this, the first is some people believe that you don't need to add bacteria to a tank because everything you need is in there, and it is true, however, as i said these bacteria in these bacterial mixes are very efficient at their chosen jobs, and adding these on a constant basis means you have the msot effcient strains of bacteria for waste assimilation and nutrient reduction.
Then in most cases you need a skimmer to then skim them out, however, i run this in my nano tank without a skimmer, and in freshwater systems and run VERY low level of solid waste, and close to 0 nitrate, phosphate is quite low at about 0.3, which is low for a skimmerless 25 liter marine tank.
So in the end, cycling a tank can be done WITHOUT bacteria, (autotrophic that is, such as nitrosomonas, nitrobacter, nitrospira, nitrosospira, nitrococcus nitrosococcus) but it is much faster with these bacteria, and seeds your tank with the right bacteria from the start, kind of a supercharge.
The heterotrophic bacteria (or probitoics) you can also do without, but, in my experience it definitely helps, and a lot of people use them. defintitely worth looking into. When doseing pro-biotics in marine aquariums, many people also dose with an Organic carbon supplement, (not activated carbon), this is a food source for bacteria and helps them to take up more nutrient, in a new tank this is usually not necessary and you would usually not add an organic carbon source unless you had a bad nutrient problem and/or wanted to go to a ultra low nutrient system.
Organic carbon dosing however is not for the faint hearted and a lot of chemistry is involved.
but, adding beneifical bacteria is none the less, beneficial, in my opinion and experience.
G
