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Aeration and dissolved oxygen: bubbles, circulation and gas exchange

Low dissolved oxygen can become an emergency quickly. Learn how gas exchange, temperature and waste affect the supply.

2 min readUpdated September 25, 2026

About this guide

Water quality

By Fishician

Published September 8, 2026

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The short answer

  • Aeration supports gas exchange. Oxygen can dissolve from bubbles as they rise. Bubbles also circulate water and disturb the surface.
  • Check circulation and the fish, and measure dissolved oxygen when possible. Bubble count alone does not show oxygen concentration.
  • Heat hits twice. Warmer water holds less oxygen while fish and bacteria need more. Salt lowers it too.
  • Low oxygen is a symptom. Overstocking, overfeeding, decay, a bloom or a stalled filter are the usual causes.
  • Fish gasping at the surface? Increase aeration promptly while checking temperature, ammonia, nitrite and equipment. Similar signs have different causes.
A disk air stone releasing a column of bubbles in an aquarium
An operating air stone moves water and helps promote gas exchange at the surface.Photo: Air Stone Disk aquarium Germany 2024 — Nimmerwelk · CC BY-SA 4.0. Resized for display.
What moves oxygen up or down
Raises oxygen
  • Surface agitation and turnover
  • A clear surface, no heavy film
  • Cooler water within the species range
  • A second, independent flow source
  • Less organic waste
Lowers oxygen
  • Rising temperature or salinity
  • Heavy stocking and feeding
  • Decaying food, plants or a dead fish
  • Algae or bacterial blooms
  • A stalled filter or sealed lid

Plants and coral help only in the light

  • Photosynthesis produces oxygen in suitable light. Respiration uses oxygen both day and night; photosynthesis stops in darkness.
  • In planted or algae-rich systems, oxygen often reaches its daily low before lights-on. Maintain gas exchange overnight.

Reading distress

  • Signs: gathering at the surface or filter outflow, fast breathing, flared gills, lethargy, not eating.
  • Several species at once? More likely the environment than disease.
  • Worse before lights-on? Points to the day-night oxygen cycle.
  • Started after heat, a power cut, cleaning, a treatment or a big feed? Look there first.
  • Adding surface movement is a safe first step while you investigate. It buys time; it doesn't fix the cause.

Build in resilience

  • Don't depend on one device. A second powerhead or an air pump on its own circuit keeps oxygen moving if a filter stops.
  • Maintain ventilation and surface movement while keeping a secure lid for jumping fish.
  • Don't let waste pile up so much that one cleaning releases a big oxygen demand.
  • Keep a way to move water by hand for power outages.
  • In heat waves, add surface movement and deal with the temperature.

Common hidden causes

  • A dead fish nobody has found yet.
  • A lot of decaying material stirred up during cleaning.
  • An algae or bacterial bloom, or a biofilter working hard through an ammonia spike.
  • A filter or powerhead that quietly stopped.
  • Adding an air pump without finding the cause leaves the problem in place.

Sources

Each source is listed with the part of this guide it supports.