If you’re thinking about starting an aquarium there’s quite a lot of info to take into account.
There’s a lot of info to be found about cycling, which is the act of getting the nitrogen cycle to complete.
Your aquarium is to be a slice of nature, in which there’s a working balance of stuff eating stuff which sets up a whole balance that lets all life flourish.
It all starts with proteins, which are excreted by fish as organic waste and is also a part of decomposition of organic materials like plants, food remnants and dead fish.
Proteins are broken down into ammonia (NH3), which is toxic to fish.
Ammonia gets converted by nitrosomonas bacteria to nitrite (NO2), which is also toxic, and then by nitrobacter bacteria into the less harmful nitrate (NO3).
Nitrate is used as fertilizer by plants who are used to produce oxygen and act as food themselves, thus completing the full circle. Nitrate also gets broken down by anaerobic bacteria that live in the filter and your substrate, which is why it’s important to clean your filter the right way so as not to destroy all the positive bacteria in the process of cleaning.
An excess of nitrate can be fixed by frequent water changes (an average of 10-20% of the total water capacity of your tank every week) and by adding more plants.
When you start a new tank it takes 36 days for the whole cycle to set, which is why a lot of people advise against adding fish for this duration, because you can get spikes in for example the nitrite which could kill your fish off.
On the other hand, adding a few hardy fish actually helps to kickstart the cycle due to the fact that they eat and excrete stuff necessary for the whole circus.
There’s a lot of products out there that supposedly add all the right bacteria to your new tank straight away, and if you look around the web you’ll find a lot of people claiming it’s bogus and a lot of people claiming they do work wonderfully. Your guess is as good as mine, but considering how much you’re spending on the whole setup with aquarium, decor, plants, fish & equipment… another $5 for a bottle of miracle bacteria won’t kill you so why not try.