Ecosystem cycles
Ecosystem cycles: how an aquarium actually works
An overview of the five interlocking loops — nitrogen, oxygen, CO₂, water, and the new-tank break-in — with interactive SVGs and links to each in-depth article.
A freshwater aquarium is five interlocking loops running in parallel. Nitrogen accumulates and is removed. Oxygen swings with the photoperiod. CO₂ flows in and out of the water. Water itself moves through the filter and out through evaporation. And the biofilter takes weeks to populate before any of the above stabilizes.
This page is the map. Each cycle has its own in-depth article with the underlying chemistry, the relevant microbial ecology, and references to the literature. Pair them with the water chemistry overview and the calculators when you need to translate readings into action.
Nitrogen cycle — 氮循环
Two-step bacterial oxidation (NH₃ → NO₂⁻ → NO₃⁻) is the engine of every freshwater tank. Read the mechanism, the literature on which bacteria actually do the work, and how to read test-kit numbers.
Hover or focus a step to read what it means in your tank.
Oxygen cycle — 氧气循环
Dissolved oxygen rises with photosynthesis, falls with respiration, and exchanges with air across the water surface. The day-night swing is the variable most beginners miss.
CO₂ cycle — 二氧化碳循环
Carbon is the limiting nutrient in most planted tanks above moderate light. Understand the carbonate-bicarbonate equilibrium, drop-checker math, and the trade-off with surface gas exchange.
Hover or focus a node to see how CO₂ moves through air, water, plants, and equipment.
Water cycle — 水循环
Evaporation, top-off, water changes, and internal circulation form one budget. The hidden consequence: mineral accumulation drifts your hardness over months if you only top off.
Hover or focus each part of the loop to see how it affects circulation and maintenance.
New-tank cycling — 新缸养水流程
Building the biofilter takes 4–6 weeks of bacterial colonization, not 7 days of waiting for water to clear. Track the ammonia → nitrite → nitrate curve with literature-backed thresholds.
Hover or focus a stage to see what happens, what to test, and what equipment you need.
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