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.

15 Gallon High – Betta Sorority at its Worst

Every tank has bad days, where plants, scaping, or fish are far from their best. Here is my 15 gallon high betta sorority tank on one of its worst days – after a huge loss of plants and an intense battle between the bettas over hierarchy upon the death of the previous queen.

My watersprite had formed such a dense, thick forest that light was unable to reach the bottom regions of the tank. All of the lower plants began to rot, as did the bottom portions of the watersprite itself. The rotting caused tank parameters to worsen, quickening the decay of all the other plants in the tank. Within a week, this tank when from a thick jungle to a barren landscape. For a betta sorority, the sudden loss of shelter was a dangerous situation, which did not end well.


All four female bettas after the battle for ranking. Now they’re fine together and show no signs of aggression toward each other. That was not the case earlier on this day.

This tank has four bettas, all seen together in the above photo. At the beginning of the week there had been five. The fifth was my first female betta, who I purchased when rather old and full-grown. She was the ruler of the tank, keeping the order among the sorority. When she passed, the remaining four bettas were left to establish a new hierarchy.

In a properly setup betta sorority tank, there would be so many hiding spots and broken lines of sight, that the fighting for dominance would mostly end at a chase. With the sudden loss of dense vegetation to provide cover, this battle for ranking became brutal. During a single workday, the bettas had a brawl that left me horrified upon returning home. Here are the results from that day.

The New Sorority Ranking – Post War of Tank Dominance

First Place – Red Betta

This darling gal is the top dog in the betta tank. She’s finally in charge after years of having to play second fiddle to the previous queen of the sorority: the late but once great betta who passed earlier this week. At first look, she seemed largely unharmed. Then I saw her left side. I’d never seen damage like hers before – it was like she’d been punched in the eye by a betta fin of steel. Getting a photo of it was ridiculously hard, but the swelling can be seen in the photo with the blue arrow pointing to the left side of her face.

Second Place – Blue Crowntail Betta

This little lady was added to the tank when quite small – less than an inch and barely old enough to sex easily. She was originally the most skittish and shy, always hiding BUT willing to fight off or chase away anyone who came near her hiding spot. This week, she decided to play hard in the fight for tank dominance, receiving the most extensive fin damage of any of the bettas. Definitely not the winner, but second best was enough to make her feel confident being out in the open all the time. No more hiding for this gal.

Third Place – Red and Blue Betta

This newcomer to the tank suffered only a gash on the top of her right front side. The scale were stripped, which could have either happened in a fight or while fleeing from one. She largely ignored the battle, but wasn’t willing to let the fighting alter her schedule of searching for food or swimming about in the open, hence the damage. I’m still not sure she knew there was a fight for ranking, but somehow she still managed to keep out of last place.

Last Place – Blue-Eyed Pink (Red and White) Crowntail Betta

This little lady spent the battle out of sight and out of mind. She doesn’t have a scratch on her, and would rather just ignore a flaring female and swim away than start a fight. This makes her my favorite in many ways – that and her beautiful tail and eyes! She’s a darling who will eat from my hand calmly. I’m glad she didn’t get hurt.

Aftermath of the Aftermath

Since the fight, I watched all of the bettas closely for signs of infection or worsening health. Water changes were performed daily to keep the tank as clean and conducive to healing as possible. Any further flaring or signs of aggression resulted in a betta timeout in the clear plastic cage on the left side of the tank photos. No further battles have been had in the two months since this original battle, and everyone has settled into their new spots well.

Betta sororrities can be difficult to maintain and start! You really need to have four or more bettas together to spread out the aggression and keep a healthy hierarchy structure. If I had been home when this was happening, I would have been able to add more decor etc to the give more hiding spots and broken lines of sight, maybe preventing some of the injuries. Had the aggression not already subsided by the time I got home, I would have changed the stocking of this tank. Every betta sorority will have some fighting/chasing when establishing the rank, but keeping it to a minimum and safe level is the job of the tank owner.

4.12 Gallon Update – Recovery Period

low shot

This tank had a rock that reacted with the water, leeching something rather toxic into the tank. I lost all but one blueberry shrimp in the end. I have since re-purposed the tank for a single pea puffer, but here are some shots of the beginning stages of recovery for this tank after it crashed upon removing the rock and performing some intense tank maintenance/re-scaping.



This tank obviously has a lot less going on than its previous iteration. I did a heavy cleaning of the top layer of substrate (while the shrimp were moved to a calmer, temporary container) to remove any lingering toxins that might have seeped into the Fluval Shrimp Stratum. There are also fewer rocks, more open space for water circulation, and fewer plants overall. I took out any plants that were suffering or blocking light, adding them to a different tank instead.



I also added in a baby red wag platy that had been shipped to me as a bonus surprise with a plant/shrimp order. It was originally in with my red cherry shrimp, who were breeding prolifically. When it became large enough that baby shrimp were on the menu, I moved it to this tank. This red wag will be going to the fish store soon enough, because I don’t have a good home for him as he gets larger.



The blueberry shrimp are doing much better now without the oxidizing rock! They’re out all the time now and quite active. It’s great to see them finally happy in their environment. At the time, I wanted to get a few more of them and try breeding, but I wound up opting for a pea puffer tank instead. The remaining blueberry shrimp were added in the the orange sunkist shrimp. Since the blueberry are neocaridina and the orange sunkist are caridina, the two species will not be able to breed with each other. Thus I still get to keep both types in a healthy environment without worries about interbreeding of colors.



I like how open and simple this tank was. These aren’t the best shots of it, but they still get across the general idea I had for the scape. The rocks and various plants on the left create one focal point, while adding a line of slight from the back left corner toward the front rocks. This leaves the far right open for the water sprite to look like an isolated tree. Definitely not my best work, but it was fun to play with the idea. Most importantly, the setup gave me plenty of control and visibility to ensure that recovery would go smoothly.