Systematic classification of nitrogen management in aquariums and explanation of this theory's mechanism
1.1 The Nitrogen Problem in Aquariums
Classification of Nitrogen Management Methods
Methods for processing ammonia (NH₄⁺) generated in aquariums can be systematically classified along a single axis: "export nitrogen outside the system" or "cycle it within." The following tree diagram shows the positioning of each method.
Fish Excretion → NH₄⁺ (Ammonia) Generated
Methods are classified by how they ultimately handle nitrogen
Note: On the Nitrification Cycle (NH₄⁺ → NO₂⁻ → NO₃⁻)
The nitrification cycle is fundamental to aquariums, protecting livestock by converting highly toxic ammonia into relatively harmless nitrate.
ただし、it only changes the form of nitrogen without altering the total nitrogen in the system. Therefore, it is not classified under "Export" in the tree diagram.
Among these methods, this theory adopts the pathway classified as "Cycle — Heterotrophic (Organic Carbon)." The following sections explain its mechanism step by step.
1.2 Direct Ammonia Assimilation by Heterotrophic Bacteria
The differences between the nitrification pathway in conventional tanks and the carbon assimilation pathway proposed by this theory are shown below.
× Conventional Tank (Nitrification)
NH₄⁺ (Ammonia)
↓ Nitrifiers (Nitrosomonas)
NO₂⁻ → NO₃⁻
↓ Releases H⁺
Continuous pH decline, nitrate accumulation
→ Regular water changes needed
◎ Holobiont Method (Carbon Assimilation)
NH₄⁺ + Carbon source
↓ Heterotrophic bacteria (C:N > 20)
Direct incorporation into bacterial protein
↓ No H⁺ released
Stable pH, zero nitrate
→ No water change needed
1.2.1 C:N Ratio and Assimilation Conditions
Which pathway heterotrophic bacteria operate on is determined by the C:N ratio (carbon to nitrogen ratio) in the environment.
C:N Ratio and Bacterial Operating Mode
🔵 Nitrification Mode C:N < 20
─ Threshold ─
🟢 Assimilation Mode C:N > 20
NH₄⁺ → NO₃⁻ (H⁺ released)NH₄⁺ → Bacterial protein
In other words, by adding carbon sources to sufficiently raise the C:N ratio, it becomes possible to switch the ammonia processing pathway from nitrification (oxidation) to assimilation (bacterial incorporation).
1.2.2 Growth Rate Differential — Why Nitrification Is Bypassed
Another condition for this pathway switch is the growth rate differential between heterotrophic bacteria and nitrifiers.
Bacterial Group
Generation Time
NH₄⁺ Processing
Byproducts
Heterotrophic bacteria
~20 min to hours
Assimilation (incorporated into bacterial protein)
CO₂, bacterial biomass
Nitrifiers (Nitrosomonas etc.)
~12-24 hours
Oxidation (NH₄⁺ → NO₂⁻ → NO₃⁻)
H⁺ (→ pH decline), NO₃⁻
Under conditions with sufficient carbon sources, heterotrophic bacteria proliferate orders of magnitude faster than nitrifiers and consume available ammonia first. As a result, the nitrification cycle (NH₄⁺ → NO₂⁻ → NO₃⁻) is effectively bypassed.
※ Kinetic premise: Heterotrophic bacteria grow faster than nitrifying bacteria. As long as sufficient carbon sources (C:N ratio ≧ ~5-10) are maintained, ammonia is assimilated before being nitrified. This competitive relationship is the central assumption of this theory.
Direct Assimilation Flow (Summary)
NH₄⁺ (Fish excretion)→C:N ratio rises by adding carbon source
Heterotrophic bacteria proliferate→Assimilate NH₄⁺ as nitrogen source into bacterial protein (no oxidation)
Zero H⁺ release→Cause of pH decline eliminated → pH stabilizes at substrate chemical equilibrium (pH lock)
1.3 Nitrogen Cycling through Detritivorous Organisms
1.3.1 Theoretical Principles
In this section, benthic detritivorous organisms are collectively referred to as benthos. This includes Tubifex worms, Gammarus, loaches, and others, which theoretically fulfill the same role regardless of size.
As described in the previous section, heterotrophic bacteria convert ammonia into bacterial biomass. However, this alone would cause biomass to continuously accumulate. The second pillar of this theory is having benthos consume this biomass, and then having the main tank fish consume the benthos, cycling nitrogen through the food chain.
1.3.2 Circulation Methods — Full-Auto and Manual
The method for returning benthos to the main tank is an implementation choice.
■ Full-Auto Method
BenthosTubifex, Gammarus, etc. (micro species)
TransferContinuous auto-supply via pump
PredatorSmall to medium fish
FeatureNo manual labor, 24h loop operation
■ Manual / Large Pump Method
BenthosLoaches etc. (large species)
TransferManual transfer or large pump
PredatorLarge carnivorous fish
FeatureLarge tanks, periodic management needed
In the system reported in this paper, considering the realistic size of household pumps, a full-auto method using micro-benthos (Tubifex, Gammarus, etc.) was adopted.
1.3.3 Material Cycle Schematic
The overall picture of material cycling between the main tank and sump is shown in the following schematic.
Main Tank (Display)
Fish prey on benthos
Fish grow
Waste and uneaten food generated
Overflow drops water to sump
This cycle continues autonomously, making water changes unnecessary
1.4 Consequences Derived from Theory
From the above mechanism — direct ammonia assimilation via carbon assimilation + food chain through detritivorous organisms — the following consequences are theoretically derived.
Consequence 1: Zero Feeding
Detritivores proliferating in the sump become food for the main tank fish. In the full-auto method, they are continuously auto-supplied through the pump.
Consequence 2: Zero CO₂
CO₂ is continuously produced as a metabolic byproduct of aerobic heterotrophic bacteria. This quantity may exceed gas loss from overflow splashing and supply sufficient CO₂ for plant photosynthesis.
Consequence 3: Non-Anaerobic Substrate
Detritivorous organisms (benthos etc.) continuously physically agitate the substrate, suppressing substrate anaerobic conditions (hydrogen sulfide generation). This reduces the need for resets.
pH Lock
A noteworthy consequence of this theory is the pH lock phenomenon. Its mechanism is explained by the following causal chain.
pH Lock Causal Chain
① Carbon source addition → C:N ratio rises
② Heterotrophic bacteria directly assimilate NH₄⁺ → Nitrification does not proceed
③ Nitric acid (HNO₃) is not produced → Cause of pH decline eliminated
④ pH stabilizes at the chemical equilibrium point determined by substrate buffering capacity
Importantly, the substrate is not a system prerequisite but a parameter that determines the pH equilibrium point. This theory functions as long as the sump has porous media (bacterial colonization surface), and the main tank can theoretically be a bare tank.
