Do yor search in here. Lots of ingo. Search for nitrogen cycle.
Basically it is a chemical process that turns water livable for fish, achieved through biological filtration.
First thing that appears in tank water, due to waste, is ammonia. Deadly poisonous.
You have to have ammonia converted to nitrites ( still poisonous ) and this nitrite is than conveerted to nitrates ( not harmfull in small quantities ).
This whole process of transforming ammonia into nitrates is called the nitrogen cycle, and when a tank has sufficient bacteria in the biological filtration to do this conversion systematically it is callled a chcled tank.
The process, at start takes about a month, than, when the tank is cycled, the process is continuous.
You should only add fish once the cycle is complete, it means, once all your readings give you a low nitrates count and Zero amonia and zero nitrites.
To cycle a tank throw a shrimp, for instance, in the water and let the cycle start.
Adding a fish to a new, uncyclwd tank, kills the fihs by ammonia poisoning.
We all do mistakes. We all started once. Do not despair
