Is CO2 dangerous for aquarium fish?

This probably is the main question lurking in every single person who wants to get into aquascaping or the planted aquarium hobby. Chances are most people who get interested in aquascaping already are interested in fishkeeping if not already are keeping a few fish.

Is CO2 gas dangerous for aquarium fish?

They would think of aquascaping as a part of fish keeping. Their main priority is the well-being of the fish or any other fauna in the tank and they are right in doing so. Fish should be the number one priority in this hobby. After all, the planted tank hobby was started by fish enthusiasts decorating the fish’s home with a few aquarium plants, gravel and wood to make it look more like the natural habitat of their beloved pets. The aquascape/ planted tank should be designed with the fish’ safety and comfort in mind because the entire design, no matter how intricate and beautiful, would still be just another tank for the fish to live in comfortably. The aquarium plants would still just be decorations for the fauna and without fauna, the scape would just be another grow-out tank for aquatic plants and not considered as an ecosystem. Flora and Fauna must be able to coexist and thrive in order for this to be considered an ecosystem.

Is CO2 gas dangerous for aquarium fish?

Now, the question, of whether CO2 gas is dangerous for aquarium fish, can easily be answered by one word and that is a resounding “Yes!”. Before jumping to conclusions though, understand first by reading through this article how it can be dangerous or how one can still use aquarium carbon supplementation despite its dangers. Before limiting the possibilities of the tank to a handful of aquarium plants that can survive in a low CO2 environment, please read through this article and weigh the options afterward.

Science of gases in the planted aquarium

Does water contain carbon at all? We will always find CO2 in water. It is even present in tap or even in potable water. Along with Oxygen, this is one of the gases that can easily be dissolved. With just a little splash or a little surface agitation, aquarium water can release some of its dissolved gases and absorb some gases in the atmosphere at the same time. This is called gas exchange. So as long as there is movement in the water surface, CO2 and other gases will always be present in the water column. The real danger would be if the gases get too high in concentration. Do plants release CO2? The answer is no. They absorb this gas and release oxygen. But it is not only CO2 that is dangerous to fish. Oxygen can be dangerous as well. Fish suffer and die if Oxygen is low and fish suffer and die if Oxygen is too high. Looking at it from this perspective, Oxygen levels can even be more dangerous to fish and fauna than CO2 can be.

What is "supersaturation"?

Too much gas (not just Carbon Dioxide) dissolved in water is called “supersaturation”. This is the silent killer when fish is put in new tap water. Oftentimes hobbyists would blame chlorine in tap water when in fact tap water is just oftentimes super saturated with gas especially if there is high pressure.

CO2 injection or supplementation in the planted tank can be done and can be done correctly to make sure that there are no ill effects on the fish and other inhabitants. There are instructions both written and in the video to help the hobbyist learn how to do this properly.

CO2 will likely kill as fast as Oxygen does if the levels are not correct. The keyword in aquascaping is always balance. Balance not only visually but scientifically as well. If carbon dioxide levels are allowed to go higher than 30 Parts Per Million, the tank inhabitants would reach up to the surface and start to gasp for air. High CO2 levels turn the water acidic as well. They will soon perish in a few minutes if not remedied on time.

How will I know if the gas levels are too high?

Oftentimes though, the fish are the last ones to show signs. If snails or shrimps are present in the tank, they would be the first ones that try to leave. Snails can often be observed congregating near the water surface trying to get out. Shrimps do this as well. The beneficial bacteria in the filter as well as in the tank will die off too. Fish, before going up to the water surface to gasp for air can be observed swimming rather slower than usual. Their movements are calculated and there is a sense of unease. Lack of appetite can also be observed. High CO2 levels will make it very difficult for fish to breathe oxygen from the water.

Is CO2 dangerous for aquarium fish?

Prevention and Cure

CO2 drop checkers do in fact help determine the presence of CO2 in the water, that is if it is used the right way. Too many incidents have happened where this pH reagent is placed where there is no water flow and would give out the wrong CO2 readings in the tank. If placed too near the CO2 source as well it can give out wrong readings too. This accessory would also not be dependable to check if CO2 is at safe levels since the readings are one hour delayed. The green color indicated on the device itself are the CO2 levels from one hour ago. This should be enough time for the CO2 levels to rise and kill all the fish. The best way really is to prevent CO2 from rising to these levels.


Is CO2 dangerous for aquarium fish?

What to do if there is too much CO2 in your aquarium?

The high-tech aquarium is something we need to observe constantly. It's far from a self-sustaining tank where all the processes depend on the ecosystem. The high-tech equipment needs to be monitored as well as the tank inhabitants. Remember that accidents will happen. CO2 valves can fail. Timers can not turn on and off as expected. These things do happen and will happen. And one cannot always be there to make sure everything is running properly.

Emergency large water change is the best cure!

In case the CO2 levels do rise, agitating the water for a few minutes and followed by a water change of 30%-50 %will set the CO2 back to less than 30 PPM. That is given that this was noticed on time.

So, the aquascaper has 2 choices; either go with a non-CO2 tank and be content with the fish and a limited type of low light-loving aquatic plants in the tank or come up with a fail-safe method that will ensure CO2 levels will not reach dangerous levels.

Are CO2 tanks dangerous themselves?

Many people decide to use liquid carbon dioxide instead of pressurized CO2 systems as they think that the leak from the cylinder can kill the fish. It's not true! In case of any leak outside the aquarium, CO2 will dissolve readily in the air, not harming the fish, or humans in the room. One thing related to the CO2 cylinder can be dangerous - the end-tank dump. This is as the tank pressure falls below a certain limit the regulator stops regulating and dumps the rest of the tank contents. This is easy to avoid with the CO2 regulator with the dual-stage construction.

Look into our offer of the best Dual-Stage regulators and keep your fish safe from end-tank-dump!


What about an air stone?

Some aquascapers run an air stone in their hi-tech planted tanks at night. This will not only ensure that enough Oxygen is going to be present in the tank, it will also ensure that carbon for fish tanks can escape to the atmosphere, therefore reducing the risk of build-up of this gas. Running the air stone in the daytime will create too much surface agitation allowing CO2 to escape that may otherwise be utilized by aquarium plants during photosynthesis.

Is CO2 dangerous for aquarium fish?

Hey, there are still skimmers!

Surface skimmers and fans can help as well. Skimmers can create agitation by sucking the surface water and creating a vortex. Fans used to lower water temperature can create surface agitation as well. Others would position lily pies so they can create a vortex, in consequence creating surface agitation. Too much surface agitation though will negate the use of CO2 injection.

Electrolysis devices set on timers can help ensure enough oxygen is present in the aquarium water at all times as well as help rid the water of unwanted organisms like free-floating algae spores.

Adjust the lighting!

A good method would be to limit lighting to just enough for plants to photosynthesize. Why is this a good method to prevent too much CO2 in the aquarium water? High lighting would force the plants to need more CO2 during photosynthesis. So, one would need to provide more CO2 making the levels dangerously close to the point where it does not kill fish instantly it would create health issues in the long run. One simple mistake in adjustment would kill all the fish. Lighting the tank in moderation would mean CO2 injection in moderation as well. So is CO2 harmful to fish? Ensure the right conditions and all you will see is the advantages of the CO2-enriched planted aquariums.

Back filters and sump filters

Using Hang on Back and Sump filters instead of canister filters would be a good practice. There is a myth that discourages the use of these types of filters and promotes the expensive canister filters in aquascaping. Yes, canister filters are great filters but it does little or nothing at all in terms of gas exchange and ensuring gases will not build up to toxic levels. Hang-on back filters and Sump filters ensure enough gas exchange 24/7. They may look odd, especially in an aquascaped tank but that is only because we have been so used to looking at aquascapes with only lily pipes attached to them. Some argue that these types of filters will only allow the precious CO2 gas to escape and the plants will have very little left to use. That only happens if there is not enough CO2 injected into the tank in the first place. The CO2 injection only needs to be adjusted a few bubbles more to provide enough for the plants. When using these filters, one would also need to ensure that the CO2 can circulate all throughout the tank before the excess can be released into the atmosphere through these filters. So, water circulation is very important when using CO2 in your aquarium.


The best method really is observation. Monitor the CO2 level in your drop checker or perform liquid ph test kits. Observe the fish daily. Note changes in the behavior and appearance of the fish. It is not uncommon to see hobbyists enjoy looking at their tanks while making assessments at the same time.

Remember about the before-mentioned balance. With this factor, you will see CO2 benefits shortly!