Nitrogen Cycle
Fish produce ammonia (NH3) from gills and waste. Ammonia is toxic at any concentration above 0.1 ppm.
Nitrosomonas bacteria convert ammonia to nitrite (NO2). Nitrite is also toxic.
Nitrobacter bacteria convert nitrite to nitrate (NO3). Nitrate is much less toxic; safe up to 40 ppm in most fish.
Plants and algae uptake nitrate as fertilizer.
Water changes export remaining nitrate from the system.
Bacteria Colonies
Beneficial bacteria colonize surfaces — substrate, filter media, decor, plant leaves.
A new tank takes 4–6 weeks to establish enough bacteria to support a fish load (the "cycling" period).
Filter media is the highest-density bacteria habitat. Never replace all filter media at once — you crash the cycle.
Bottled bacteria (Seachem Stability, Fritz Turbo Start) accelerates cycling but doesn't replace it.
Plant-Bacteria Symbiosis
Plants prefer ammonium (NH4) as nitrogen source — the same form that's toxic to fish. They convert it directly without bacterial help.
In heavily planted tanks, plants outcompete the bacteria for ammonia, processing it before bacteria get the chance.
Result: tanks heavy with fast-growing plants barely need biological filtration.
Algae Competition
Algae and plants compete for the same nutrients (N, P, K, light).
When plants thrive, they outcompete algae. When plants struggle, algae wins.
Forcing balance: heavy planting + appropriate light + nutrient sufficiency.
Carbon Cycle
CO2 produced by fish respiration and bacterial decomposition is consumed by plants during photosynthesis.
Plants release O2 during photosynthesis (daytime) and consume O2 at night.
Stable tanks self-balance CO2 and O2.
Detritus and the Substrate
Fish waste, decaying plant matter, and uneaten food sink to the substrate.
Detritivores (worms, snails, certain shrimp) break it down.
Bacteria mineralize it into plant-available nutrients.
A "dirty" substrate in a planted tank is actually a nutrient bank.
Self-Sustaining Tanks
A truly self-sustaining tank: heavy plant mass, light fish stocking, balanced fish food, minimal external intervention.
Water changes can drop to monthly or quarterly. Some Walstad tanks run 5+ years on minimal changes.
The closer to balance, the less work you do.