An air pump brings air and water into contact
The bubbles rising through an aquarium contain ordinary room air, roughly 21% oxygen. A common electromagnetic diaphragm air pump neither splits water nor removes nitrogen.
A changing magnetic field drives a magnet and a flexible diaphragm back and forth. As the diaphragm withdraws, the chamber expands, its pressure falls and the inlet valve opens. As it presses in, the chamber contracts, pressure rises and the outlet valve opens. Alternating one-way valves turn a reciprocating movement into an outward air supply. Our section shows one chamber and greatly slows the movement. Real products also include twin-diaphragm and other drive arrangements.
A hose delivers air to a porous air stone underwater. Air must overcome the pressure of the water above it and resistance in the diffuser before bubbles emerge. The stone divides air into bubbles; it does not manufacture oxygen.
Oxygen crosses the bubble–water interface. Rising bubbles also move water and renew contact at the water surface. Both routes contribute. Their relative importance depends on bubble size, depth, airflow, geometry and circulation; no fixed percentage applies to every aquarium. Gold dots stand for oxygen dispersed at the molecular level, not invisible tiny bubbles floating in the water.
Watch first, then change one input
In exploration mode, change relative airflow, the teaching bubble diameter and oxygen consumption. Compare the model’s oxygen concentration after four hours. At zero airflow, the air tracers and bubbles disappear together. A background exchange term remains at the water surface.
In the bubble chapter, one sphere becomes eight spheres with half its radius. Total gas volume stays the same while total surface area doubles. This geometric comparison does not imply that real oxygen transfer or electrical efficiency necessarily doubles.
In the stop chapter, both curves start with the same dissolved oxygen inventory. One keeps the pump running; the other stops it at the start. Stopping changes the subsequent rates: it does not instantly remove oxygen already dissolved in the water. These curves represent invented teaching conditions, not aquarium predictions or animal-care advice.
Dissolved oxygen follows a balance
Why do more bubbles not make oxygen rise forever?
Net transfer depends on the distance from equilibrium. As dissolved oxygen approaches equilibrium with air, net entry slows. Under supersaturated conditions, oxygen can also leave the water. Equilibrium concentration varies with temperature, salinity and pressure. Respiration and decomposition consume oxygen; photosynthesis can supply it.
The film fixes temperature and consumption and omits spatial differences, photosynthesis, changing oxygen partial pressure inside bubbles and hose transients. 8.3 mg/L is an approximate air-equilibrium example for freshwater near 25 °C at sea level. The scene sets no species-specific safety threshold.
How are the curves calculated?
C is dissolved oxygen in mg/L, C* is the concentration in equilibrium with air under the stated conditions, k_b and k_s are lumped bubble and surface transfer coefficients in inverse hours, and R is consumption in mg/L/h. The coefficients bundle complex interface and mixing effects into uncalibrated teaching parameters. They are not measurements of a commercial device.
For fixed inputs, the exact solution reconstructs any point in time, with concentration bounded below by zero. Once oxygen is depleted, actual consumption cannot exceed the remaining inventory plus incoming oxygen. Cumulative transfer, consumption and inventory obey the same balance. Stopping removes the bubble term and reduces, but does not eliminate, surface exchange.
Technical references: Thomas Pumps on linear pump operation, the Tetra APS manufacturer manual, USGS on dissolved oxygen, and EPA on fine-bubble aeration.