Build biological filtration without exposing livestock to ammonia or nitrite.
A biologically ready aquarium can fully convert a measured ammonia dose into nitrite and then nitrate inside a repeatable time window. The point is not simply seeing nitrate on a test strip. The point is predictable oxidation capacity that matches the stocking plan you intend to introduce.
For most tropical community starts, dose to 2 ppm total ammonia nitrogen. Test after dosing, then monitor daily. Do not keep redosing randomly while nitrite is high. You are trying to establish stable colonies, not chase movement on a graph.
The most common reasons are low temperature, low alkalinity, overdosing ammonia, and poor bacterial seeding surfaces. Keep temperature warm, maintain KH so nitrification does not crash, and ensure the filter contains porous media with strong oxygen exposure.
Your system is ready when it can clear the planned ammonia dose inside twenty-four hours and both ammonia and nitrite test at zero before the final large water change. Only then should you reduce nitrate and begin stocking in a disciplined staged sequence.