How Long Does It Take Hydrogen Sulfide to Kill Fish in an Aquarium?

All aquariums have their own little ecosystems and produce beneficial and harmful byproducts such as ammonia and nitrates.

Unfortunately, aquariums can also produce gases such as hydrogen sulfide, which is extremely harmful.

Hydrogen sulfide can kill fish based on the concentration of the gas.

Higher concentrations of Hydrogen sulfide can kill your fish within 5 minutes of exposure. Lower concentrations might take 30 minutes to 12 hours of continuous exposure. Unless appropriate action is taken, there is little to no chance of saving your fish.

What is Hydrogen Sulfide?

Hydrogen sulfide (h2s) is a toxic, flammable gas that is naturally produced. It is colorless and denser than oxygen but it does have an odor.

At most, you will notice a smell similar to rotten eggs. Hydrogen sulfide commonly occurs in well water, or in an area where organic matter is broken down.

The absence of oxygen causes organic matter to use hydrogen and it produces hydrogen sulfide. It can also be produced in sewers or swamps.

When present in high amounts, this gas is toxic and is known to kill any living creature.

Fish, small animals exposed to it can die within minutes or experience extreme side effects that can put their health in danger.

How Can Hydrogen Sulfide Come into an Aquarium?

In aquariums, hydrogen sulfide is created by the anaerobic bacteria that live within the substrate of the tank.

In small quantities, this gas releases itself from the substrate in the form of bubbles. These can be noticed as appearing from no particular source.

It is the most common sign that this gas is present. This then means that the substrate must be handled carefully.

In small amounts, the gas simply oxidizes once it reaches the top of the water. However, if the substrate is disturbed too much, large amounts of the gas could be released. This could prove to be harmful or lethal for your fish.

In certain circumstances, hydrogen sulfide can even be produced in the filter of the aquarium.

Bio-filters can house anaerobic bacteria, which could then start to produce this gas.

When present in filters, fish are more likely to die as hydrogen sulfide keeps being added to the water.

When the rate of oxidation is lower than the rate of production of hydrogen sulfide, it will be present in large amounts.

This will then start to kill the fish that are exposed to it.

How Does Hydrogen Sulfide Kill Fishes?

Hydrogen sulfide kills fish by interfering with their breathing function. It transforms the iron content in the blood of the fish by turning it into sulfide.

This process restricts oxygen absorption and increases respiratory stress. As a result, the fish start to suffocate and will eventually die.

Lethal concentrations of hydrogen sulfide for fish come up to 50 μg/L. At this concentration level, there is no chance of survival.

Exposure to lower levels such as 20 μg/L is known to cause extreme stress to fish.

Unfortunately, naturally delicate fish species will not survive exposure to even 20 μg/L of hydrogen sulfide in the tank.

At the most, even the healthiest of fish will tolerate exposure to5μg/L only without any negative side effects.

Anything higher than this number and your fish will either start to die.

Symptoms of Hydrogen Sulfide Poisoning in Fish

The biggest sign that you have hydrogen sulfide poisoning is that the fish will start to gasp for air near the surface.

Unfortunately, since hydrogen sulfide is denser than oxygen, it will form a thick layer above the water as well.

The fish will be unable to breathe and within minutes, they will start to die.

Other symptoms include the following:

  • Lethargic swimming – unable to swim with energy
  • Catatonic states at abnormal times – Fish enter this state when they are sleeping only
  • Stress signs – Flaring of fins, loss of color
  • Red gills – Fish will reveal their gills to try and get more oxygen

Unfortunately, not all aquarists understand that these signs are caused by hydrogen sulfide poisoning.

That’s because they are similar to signs fish display when nitrate levels are too high, pH level or when the water temperature is not good.

Due to this, quickly identifying the source can be difficult then. However, it is better to assume the worst and treat your fish as if they are experiencing hydrogen sulfide poisoning.

Some people have been lucky enough to apply the appropriate measures and save their fish. But many, especially large community tank owners have not been as lucky.

Undetected hydrogen sulfide outbreaks have wiped entire tanks before anyone has been able to notice that something is wrong.

Saving Your Fish in Time

Sometimes, it is possible to save your fish, especially if you spot the symptoms in time.

The best thing to do is to remove the fish and move them into another aquarium.

This is necessary, as you will have to clean your substrate to get all the gas out of the aquarium.

As the substrate gets disturbed, larger amounts of gas will start being released.

This means that even if your fish survived the initial release of gas, they might die when you are trying to clean up the tank.

If you do not have another aquarium available, just take a bucket, add some dechlorinated water, and pop your fish in there.

Add the water heater and bring the water up to the temperature in the bucket. Your fish will be lethargic at first but they will recover.

Recovery from Hydrogen Sulfide Poisoning – Fish that Survive

Unfortunately, recovery from hydrogen sulfide poisoning is going to be a slow process, depending on how much of the gas the fish were exposed to.

While they may survive, their immune system is greatly weakened in this state. Great care must be taken, as they will be susceptible to diseases.

Monitor the water parameters constantly. Any additional stressors can kill the fish at this point.

Studies have shown though that shrimps and invertebrates tend to be more hardy than fish in this situation.

Shrimps have been known to be able to survive in tanks with a 70% concentration of hydrogen sulfide. Fish are more sensitive though.

Keeping the water in pristine condition, maintaining proper temperature and a good steady diet will allow your fish to recover slowly but surely.

Your fish should be healthy and in good condition within a week or two.

It is a good idea to keep your fish in a quarantine tank, especially if you are thinking of adding some fish medication.

Fish supplements can also help to speed up recovery but be very careful about using them.

The wrong dosage can accidentally kill your fish or ruin its chances of survival as well.

It’s Harmful to You Too

Did you know that hydrogen sulfide is also harmful to humans?

While it is not found in large concentrations in an aquarium, people who have been exposed to as much as 600 mg of the gas have experienced severe poisoning.

As a toxic gas, hydrogen sulfide must be handled with care.

If you are looking after multiple fish tanks or extremely large-sized fish tanks, make sure that you are in a well-ventilated area.

Poor ventilation can make the gas more concentrated.

While there is no proof of anyone being poisoned by their aquarium, it is better to exercise caution in this area.

Healthy Ways to Keep It Out of Your Aquarium

Hydrogen sulfide can be kept out of your aquarium by applying good aquarium upkeep practices.

The following are the top ways you can keep it out of your aquarium:

Vacuum Your Gravel

Always vacuum your gravel, either on a monthly or weekly basis.

Hydrogen sulfide is caused by anaerobic bacteria, which reside in the sediment gravel. As the gas is produced, it accumulates in the gravel.

Other debris such as fish waste and food tend to collect here too which can work as fuel for the anaerobic bacteria.

There can be around 100mg/L of hydrogen sulfide gas in some substrates. By vacuuming the gravel, you can effectively release the gas before it accumulates to lethal levels.

This ensures that it does not harm your fish.

Use the Right Amount of Substrate

Too much substrate can contribute to the production of hydrogen sulfide in the aquarium.

It is recommended that aquariums should have no more than 1 inch of substrate. Thicker substrates trap more waste and debris and they prevent any release of gas.

This means that when the substrate is disturbed, extremely large concentrations of the gas can escape.

Large concentrations of hydrogen sulfide are harmful and they will end up poisoning the fish in your tank.

If you are using a thicker substrate, weekly vacuuming is going to be necessary.

Have Live Plants

One of the best ways to prevent hydrogen sulfide gas production is by getting live plants in your fish tank.

Live plants have roots, which properly aerate the substrate. As the roots grow, they prevent the substrate from becoming packed or stuck in one place.

The plants also help to prevent any residue from being collected in the substrate.

This ensures that the anaerobic bacteria do not get a source that they can use to produce hydrogen sulfide.

The more plants you add, the healthier your tank will be. Additionally, your fish will love the plants too.

Move Your Décor Pieces

Make sure to move your décor pieces around from time to time.

Hydrogen sulfide gas can collect under the rocks, décor pieces, and toys you placed in the tank for your fish.

By moving them around properly, you can prevent the gas from being trapped under there in large concentrations.

This does not mean that you reshape your tanks every time you are cleaning them.

If you like the layout of the tank, you can still lift up the décor pieces to allow the gas to escape.

This will be enough to release the gas and prevent larger concentrations.

With these helpful tips, you can easily ensure that your tank stays free of hydrogen sulfide and your fish remain healthy.

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