The Nitrogen Cycle Explained
Fish produce ammonia, which is toxic. Beneficial bacteria convert ammonia to nitrite (also toxic), then to nitrate (far less toxic), which you remove with water changes.
Cycling a new tank means growing those bacteria before adding fish — typically 2-6 weeks. Test for 0 ammonia and 0 nitrite before stocking.
Overstocking overwhelms the bacteria and causes ammonia spikes. That is why a sensible stocking level matters as much as filtration.