Hello and welcome, the most important process in setting up your new aquarium is establishing the biological filtration, it takes the ammonia generated by fish waste, left over food, decaying plants, ect. and converts it to a less harmful (key word is less) nitrates. The term used is cycling. here are two types.
Fishless cycling is IMO (in my opinion) the best way to introduce pure ammonia to the aquarium. The ammonia is converted to nitrItes as a byproduct of one type of bacteria and the nitrites are used as a food source by another type of bacteria and creates nitrAtes as their by product. No fish are in the tank during this method
Ammonia is generated by fish waste, decaying plants, and fish food. If there are not any beneficial bacteria to convert it by the nitrogen cycle into less harmful nitrAtes, then it will cause your fish to become sick and ultimately die.
The fish in cycle (not my choice) is by using the fish waste and food to get the nitrogen cycle going by the breakdown of them creating ammonia. Your fish will suffer and perhaps perish in the process. it can also take much longer due to the small amounts of ammonia that can be introduced because the ammonia again will harm the fish. Not the best option.
I am attaching an article for you to read and it explains the process. The choice is yours. Please don’t be insulted by the name of the article. It wasn’t intended to be insulting.
Local Fish Stores sometimes only want your money, by properly cycling your aquarium; you will save time in the long run and hopefully will cause no harm to your new pets. We don’t want your money, just you and your fish to thrive, not just survive. We will help you anyway possible.
Fishless Cycling For Dummies