Aquarium Water Chemistry and Physics Deep Guide: Solubility Diffusion
Every aquarium runs on physical chemistry that no test kit displays — gas solubility curves, diffusion gradients, ionic equilibria — and ignoring the underlying physics is why hobbyists stumble over surface agitation, KH buffering and CO2 dosing for years. Aquarium water chemistry physics is best understood as four laws applied to a glass box: Henry’s law for gas dissolution, Fick’s law for diffusion, the carbonate equilibrium for pH stability, and Boltzmann thermodynamics for temperature effects. This deep dive from Gensou Aquascaping at 5 Everton Park unpacks each, with the numbers that matter at typical Singapore conditions.
Oxygen Solubility and Temperature
Oxygen solubility in fresh water is inversely related to temperature. At 20°C, water saturates at around 9.1 mg/L dissolved oxygen. At 26°C — typical Singapore room temperature — saturation drops to 8.0 mg/L. At 30°C, only 7.5 mg/L. This 17 per cent drop between 20°C and 30°C is why tropical tanks need more surface agitation than coldwater ones to support the same biomass. A discus tank running at 30°C is operating with permanently less margin than a guppy tank at 24°C.
Henry’s Law and CO2 Dissolution
Gas dissolution into water follows Henry’s law: at constant temperature, the amount of gas dissolved is proportional to its partial pressure above the water. Atmospheric CO2 at 0.04 per cent partial pressure equilibrates water to roughly 0.6 mg/L dissolved CO2. A pressurised CO2 system pumping into a diffuser raises local partial pressure dramatically, driving 20-30 mg/L into solution before equilibrium with the atmosphere pulls it back out at the surface. This is why surface agitation gasses off CO2 — and why high-tech tanks turn off filters or close lids during the photoperiod.
Fick’s Law and Boundary Layers
Diffusion across a concentration gradient follows Fick’s first law: flux equals the diffusion coefficient times the concentration gradient over distance. In practical terms, dissolved nutrients reach a leaf surface only as fast as the concentration gradient and boundary-layer thickness permit. Still water creates thick boundary layers; flow from the filtration range compresses the layer and accelerates uptake. This is why CO2 injection underperforms in low-flow tanks even at high gas concentrations.
The Carbonate Buffer Explained
The carbonate equilibrium is the single most important pH-stabilising reaction in freshwater systems. CO2 dissolves to form carbonic acid (H2CO3), which dissociates into bicarbonate (HCO3-) and carbonate (CO3^2-) depending on pH. At pH 6.5-8.5, bicarbonate dominates and acts as a buffer — it neutralises added H+ by accepting protons to reform H2CO3, and it neutralises added OH- by releasing protons. KH (carbonate hardness) measures the bicarbonate reserve. Singapore PUB tap typically reads KH 1-2, which is a thin buffer that can crash under heavy CO2 dosing.
pH as a Logarithmic Scale
pH is the negative logarithm of hydrogen ion concentration: pH = -log[H+]. A pH drop from 7.0 to 6.0 represents a tenfold increase in H+ concentration; from 7.0 to 5.0 is a hundredfold. This is why pH crashes feel sudden — half the buffer can be exhausted with no measurable pH change, then the next ten per cent drops the reading by a full unit. The aquarium additive range includes KH boosters specifically to widen the buffer reserve in soft local water.
Surface Area and Gas Exchange
Gas exchange between water and atmosphere is proportional to surface area, not water volume. A long shallow tank exchanges gases far faster than a tall narrow one of equal volume. This is why circular bowls work poorly for fish — limited surface area combined with no flow creates near-anaerobic conditions within hours. The aquarium tank range rimless designs with broad open surfaces support far higher biomass than equivalent-volume bowfront tanks.
Conductivity, TDS and Ionic Strength
Conductivity measures dissolved ion concentration and is roughly proportional to total dissolved solids. Singapore tap water typically reads 80-150 microsiemens — very soft. RO water reads under 10. Marine water reads 50,000+. Conductivity changes act as spawning triggers for many South American characins because rainfall historically drops river conductivity dramatically. Monitor with a TDS meter for breeding work; the swing matters more than the absolute value.
Osmoregulation and Salt Gradients
Freshwater fish are hyperosmotic to their environment — their internal salt concentration is higher than the surrounding water, so water flows in passively and they constantly excrete dilute urine to compensate. Marine fish run the opposite. Brackish species switch metabolic modes. This is why sudden salinity changes are physiologically violent — gill chloride cells must reverse pump direction, and the lag costs energy and fish.
Temperature, Metabolism and Q10
Biological reaction rates roughly double for every 10°C rise — the Q10 rule. A discus at 30°C metabolises nearly twice as fast as the same fish at 20°C. This compounds with reduced oxygen solubility to make warm-water fishkeeping inherently more demanding, not less.
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