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.

AZOO Mignon Filter 60 and 150 Review

Mignon Flow

Most of my smaller tanks use AZOO Mignon filters – either 60 or 150 depending on the tank size. These little filters are quite sleek and compact, and they come with a great price tag. The AZOO 60 works best for pico tanks under 3.5 gallons. The 150 claims to be good for tanks up to 30 gallons on the Amazon.com listing, but I have mostly restricted their use for tank 15 gallons and under.

Mignon Filter

While the AZOO Mignon filters might not have the most impressive exterior packaging, they are quite well designed. The filter comes filled with the inclued intake extensions, intake sponge covering, filter sponges/cartirdges, and such. The intake has an adjustable knob, allowing for great flow control. I’ve also constructed a few baffles to change the water surface disruption levels for my floating plants.

The intake tubing is all clear, with a black filter sponge. Overall, the filter is quite unobtrusive in the tank, and the pre-filter sponge works quite well.

These filters are really capable of some powerful flow. My only complaint is that the intake extension tubes have fallen apart in the tank before. As I was using the filter on a nano tank, I came home to find a few shrimp and fish had been sucked directly into the filter. This problem has happened twice now on the same filter, leading me to replace the intake tubing on the filter and be a bit paranoid about checking that particular filter. I have five of these filters running right now, and the issue only occurred for one of the filters, and would have been avoided had I paid more attention to the tank with the problematic filter.

Riccia Fluitans/Moss Mats – Fishing Line and plastic grid/cloth method

There are many different methods to create flat sections of moss, riccia, or other plant life that doesn’t typically carpet or grow in such a form naturally. These mats can be quite effective when fully grown, and allow the easy control and use of new colors and textures in the planted tank. Takashi Amano is credited with first using riccia fluitans, aka crystalwort, in such a scenario. Since, many different types of plants have been experimented with, some with more success than others. Here’s what you’ll need to make a mat:

The Plant

Mats work best with fast growing species that split in many different directions as they grow.  Common choices include Riccia fluitans/ CrystalwortJava MossChristmas MossPellia liverwort (Monosolenium tenerum), or Fissidens.

The Mat Material

Any material which can withstand being under water without decay or contaminating the water (many plastics, stainless steel 316, etc) can bee used for this. Common choices include nylon netting from craft stores, plastic grids from craft or hardware stores, metal meshes, or re-used material from bath poofs, filter media bags,… The material needs many openings that are large enough to let in light and allow the plant to grow through while being small enough to hold back the initial portion of the plant. Be careful if using metal – stainless steel 316 is the most common choice for fish tank use, and those edges can be sharp!!


Any string or thin material that won’t degrade under water that can fit through the holes of the chosen material works for this step. Fishing line is a common choice, as it is hard to see and easy to use. The thinner the line, the less visible and easier to work with. Riccia Line is usually just a green tinted fishing line, such that it blends in quite well immediately. Nylon thread also works well for this and is available in a variety of colors. If you steel mesh for the material and attaching the mat directly to wood or an object, then thread or line won’t usually be needed.

My riccia and moss mat process using filter media bag and fishing line:

Collect the portions of riccia, moss, etc to be used and plan out the size of the mat. The plant will need to be placed between two layers of the mat material, so either cut two identical pieces for the top and bottom, else, if the material is easily folded, cut a single piece that can be folded to give the right mat shape. Giving yourself some extra room around the edges that can be trimmed can make sewing together the mat easier.

Spread the plant portion over the mat material that will form the bottom half of the mat, as below.

A uniform layer just thick enough to cover the entire mat bottom is ideal. If the layer is too thick, portions of the plant often die or decay due to lack of light and water flow. Put the top half of the mat into place, and use the thread/line to tie small knots every half inch or more along the open edges.

I keep the mat wet with tank water throughout the process. Any sections of the plant that dry out can become damaged and die rather than grow into the mat.

I’ll be sure to post updates soon on how these mats are growing out in my new 7 gallon setup! I also ordered some stainless steel 316 wire mesh/screening this week, and will be making some mat portions with that as well. I’ll post on it once I’ve found a good method.