What to do when you’ve gassed your fish – CO2 Emergency Help & Aftermath

Most everyone who has tried adding CO2 to their planted tanks has had one of those terrifying moments of realization that something is horribly wrong with their aquarium. Whether it is a faulty regulator, having too high a CO2 rate, or any of the other multitudes of ways CO2 can go awry, there are telltale signs of the potential tragedy unfolding before it is too late.

Signs of CO2 Overdose:

  1. Fish gasping for air on the surface
  2. Dead fish or inverts
  3. Normally active or outgoing fish lethargic or hiding
  4. Normally shy or bottom dwelling fish staying toward the upper regions of the tank


Once you’ve realized that CO2 is definitely the cause of your fish tank woes, there are steps you can take to save the remaining creatures and ecosystem before complete tragedy. There are two main components of CO2 gassing that cause catastrophe: the CO2 gas and the pH changes is causes. Both can kill fish, inverts, and the necessary beneficial bacteria that keep a tank ecosystem functioning. Luckily, there are ways to remedy both of these issues quickly an effectively, potentially saving your aquarium.

Oh no!!! Emergency steps for a CO2 Overdose:

  1. Turn off the source of the CO2 immediately.
  2. Agitate the water’s surface. This can be done by:
    • Adding an airstone or other bubbling aquarium component
    • Turning a pump/powerhead toward the surface of the water
    • Lifting the filter’s outflow to be above the water’s surface, so the water exiting the filter creates bubbles as it falls back into the tank
  3. Do an emergency water change.
    • If most fish are swimming at surface, gasping for air, do 2 ~20% water changes, refilling the tank after removing the 20%. This will effectively be a 35% water change.
    • If fish are breathing, but collapsed on the bottom of the tank, do 3 or 4 ~20% water changes, refilling the tank after each 20% removal. 3 will change out ~half the tank water, while 4 will change out ~60%.
    • If all fish are deceased, a larger water change is fine.
  4. Remove any dead fish/inverts.
  5. Any fish remaining on the bottom of the tank but not obviously dead can be moved to an area of higher flow or aeration. Simply scooping them into a fish net and positioning or holding the net above an airstone, at the filter outflow, or similar can make a huge difference.


Okay, so the emergency is finally over. The tank is beginning to recover and out of immediate danger. Yet there are still many dangerous repercussions after a CO2 gassing, depending on how severely the tank environment was changed. Preventative measures can avoid these entirely.

After the Emergency

  1. Figure out what went wrong and make sure it cannot ever happen again. If your equipment was faulty or unable to make a steady bubble rate, fix or replace the equipment. Don’t let it happen again.
  2. Watch for signs of a mini or full cycle of your tank again. I’ve found that adding products designed to help tanks skip the initial cycle can really help in this instance. Three of the ones I’ve tried, listed in the order I trust the product manufacturer, are:
  3. Watch for further signs of fish stress or damage. Depending on how bad the gassing was, the fish could be quite weak and sensitive for some time. Consider lowing water flows to let them rest.
  4. Monitor your pH and other tank parameters. The gassing and the possibly large water changes afterward could have upset the balance of the tank. Getting back to steady pH, hardness, and mineral levels could take some time.

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