Freshwater Aquarium Nitrogen Cycle Basics

 It’s all about how waste gets broken down and transformed by beneficial bacteria into less harmful substances. Here’s how it works in simple terms:

Waste Production: Fish and other aquatic beings produce waste—mainly ammonia—through their excrement, uneaten food, and even their breathing (via their gills). Ammonia is toxic to fish, and is the starting point of the cycle. 

Ammonia to Nitrite: Special bacteria called Nitrosomonas (or similar species) eat the ammonia and convert it into nitrite. Nitrite is still toxic, just a bit less so than ammonia, but it’s not the end goal yet.

Nitrite to Nitrate: Another group of bacteria, like Nitrospira or Nitrobacter, step in and turn the nitrite into nitrate. Nitrate is much less harmful to fish in low to moderate amounts, though it can still build up over time.

Nitrate Management: In a natural ecosystem, plants or other processes might use up nitrate. Anaerobic (not requiring oxygen) bacteria degrade nitrates into harmless nitrogen. In an aquarium, you keep it in check with regular water changes (usually 10-25% weekly), growing plants that absorb it as a nutrient, and having a part of the tank with anaerobic bacteria, which can be present deep in the tank's substrate, where water doesn't circulate much. 

When you set up a new tank, this cycle isn’t established right away—it takes time for these bacteria to grow on surfaces like the filter media, gravel, or decorations. This process, called "cycling," can take 3-6 weeks. You can kickstart it by adding bacteria sold just for the purpose and a bit of urine for ammonia, then monitor levels with a test kit until ammonia and nitrite drop to zero and nitrate starts showing up.  That’s when you know the tank is "cycled" and ready to start adding fish, shrimp or other inhabitants.

In short, the nitrogen cycle is your aquarium’s natural cleanup crew, turning toxic waste into something manageable.