Fast and dirty:
To keep dangerous contaminate level down a tank actually needs certain bacteria.
A filter does practically nothing without them. the purpose of filters is to provided palce for said bacteria to grow and a way for it to convert all the water, by a current of some type.
One bacteria, the most important to a new tank converts the most dangerous waste:Ammonia
Ammonia is not only expelled by the living as waste, it is alkso a product of dedecomposing material (food/plant/dead fish).
This first guy turns ammonia into its own waste product: nitrIte. This is also fairly dangerous.
but there is another bacteria that uses this easy supply. So bacteria number two
turns NitrIte into NitAte. Which though dangerous, need a much larger proportion to cause problems.
NitrAte spikes of any large number should almost never be seen if you make your tank water changes in a timely fashion.
This balance is not perpetual. Many things can upset it or change it. But High spikes are rarer in established older tanks of 5-6 months or more unless there has been poor water qualities kept.
Most people lose the most new fish that are in a newly cycling tank.
That's the brief and skinny