Small bubbles form on the wall of a newly filled aquarium
Small bubbles form on the wall of a newly filled aquarium

Gas Bubble Disease ("GBD") is a dangerous condition similar to the BENDS (decompression sickness) experienced by divers. GBD is characterized by the formation of gas bubbles inside a fish's body cavity. Bubbles may form behind the eyes (causing "popeye" or exopthalmia), beneath the skin, in the blood (blocking its flow and causing tissue death), or worst of all, in the gills, blocking blood flow and leading to death by asphyxiation.


While it is very serious, this condition is also completely preventable. Understanding the causes and then protecting your fish from them is very important.


One cause of gas bubble disease is the supersaturation of gases in aquarium water.  This supersaturation occurs:

1. Due to the pressure under which tap water (which in turn fills your aquarium) is pumped.
  • Tap water distribution systems maintain the water under pressure at all times, both to assure proper water flow and to prevent dirty water from outside of the pipes entering them at any leaks (the pressurized water would spray out instead).
  • Any gas (including air) introduced into the pipes will dissolve in the water at these increased pressures, and the result is supersaturated water coming from your tap.
2. When cold water warms up to room temperature.
  • Cold water can hold more dissolved gasses than warm water, and gasses are more soluble in cold.
  • As cold water warms up, it doesn't release these dissolved gasses immediately, resulting first in warm water (with a lower capacity for dissolved gases) still holding the same amount of dissolved gases - supersaturation.

Note that both of these conditions may be present when you fill a new aquarium or change water. It is at these times that we must take steps to protect our fish from GBD.


As mentioned above, supersaturated water does not immediately release the gasses dissolved in it - it needs time to go through this process of outgassing (releasing these dissolved gasses). 

  • In a newly filled aquarium, you will note that after sitting for some time, lines of bubbles begin to form on the walls (see picture above) - this is outgassing taking place, and these bubbles could form inside your fish if they were in the tank at the time.


The following will allow the outgassing process to take place:

  • Allow the water to sit, uncovered, for at least 24 hours (until the lines of bubbles mentioned above disappear)
    • Please note that allowing water to stand does not cause harmful chlorine and chloramines to evaporate off - water conditioner is still needed. See our BETTA MYTHS page for an explanation of this.


And this may help the outgassing process along:

  • Increase surface agitation, either by running the filter on your tank or adding an airstone
    • Please note that airstones do not increase the amount of dissolved oxygen in the water. Though they introduce bubbles of air into the water column, your aquarium is not pressurized and thus these air bubbles will not dissolve in the water (see causes of supersaturation above)



GBD caused by supersaturation/insufficient outgassing is entirely preventable, and all steps should be taken to avoid it.