How To Sterilize A Fish Tank After The Fish Died?

Fish death is very common in the fishkeeping journey, especially if you just started out. It transpires also to seasoned aquarists and fish may die for various reasons. But how to sterilize a fish tank after the fish died?

A fish tank can be sterilized in several ways. Sanitizing with a bleach solution or vinegar is the most typical way to disinfect the aquarium after a disease outbreak.

However, merely running the tank for a couple of weeks without any inhabitants can do the trick to kill off any parasite present.

In the below section, I’ll tell you how should you respond after having a dead fish, and what ways you can follow to sterilize the tank.

Do I have to clean my tank after a fish dies?

Do I have to clean my tank after a fish dies?

If the fish dies of contagious diseases it is required to clean or sterilize the tank. The fish is dead but the carrier of the disease is still living inside.

So, what will be the course of action you’ll need to follow in case of a dead fish?

Step-1: Remove the dead Fish & Examine

Dead Goldfish
Dead goldfish lying in the bottom of the aquarium

Whenever you found a dead fish you should immediately remove the fish from the tank, and dispose of it.

Dead fish can contaminate the water quickly which can also affect the other healthy fish in the aquarium. It will start to rot within 3-4 days of its death.

Once the process starts it can seriously damage the water parameters.

Ammonia and nitrite spike is pretty common in this case that can kill other inhabitants of the aquarium.

However, if you have a functioning aquarium filter the situation may not escalate because the filter will take care of this.

Sometimes finding the dead fish can be tricky because not all dead fish float instantly. Some even never float and decompose unnoticed in the tank.

a dead gold fish on a netting
a dead goldfish on a net

Therefore, keeping a head count can help you to maintain the inventory, and fight against dead fish contamination.

If you find the dead fish body, carefully examine it and try to figure out what could be the cause.

Check if the fish is injured, its fins are ragged, or scuffed, which means the fish was subject to bullying by other tank mates.

Also, check for white spots and a bloated belly that indicates disease symptoms.

Fish may die for several reasons, and you don’t have to sterilize the tank unless it has died from an infectious disease.

Step-2: Test The Water & decide

Testing the water can give you some clues. Every time you have a dead fish you should check the water quality.

Grab a reliable test kit and measure the water parameters. An unhealthy aquarium is one of the main causes of fish death.

Test the water
Testing the aquarium water

Check for ammonia, nitrite, nitrate, pH, temperature, etc. to ensure that something is wrong with the water. If any of the parameters is out of safe range try to adjust to the recommended zone.

However, do it slowly, any sudden change in water parameters may stress your fish.

If you suspect your fish died of bad water quality you not need to sterilize the tank. Making necessary corrections will save other fish from dying.

Below are the ideal parameters for a freshwater aquarium. But, you should set the tank parameters as per the fish species you’re having in the tank.

Water Parameters Ideal Range for Freshwater Aquarium
Ammonia 0 ppm
Nitrite 0 ppm
Nitrate <50 ppm
Temperature 72-82 °F
pH 6.5~7.5
General hardness (GH) 4~12 GH
Carbonate Hardness (KH) 4~8 KH
Oxygen Saturation >70%

If the water is just fine you need to think about other aspects as well such as proper diet, stress, etc.

In addition, watch out for other fish and how they are doing. If just one of your fish died and others are healthy that’s a good sign.

If all are acting strangely or showing unusual behavior like not eating, laying at the bottom, or gasping on the surface then it’s a matter of serious concern. I suggest you seek expert help.

Should I change the water after the fish died?

You should only change the water if the fish died of poor water quality. Sometimes fish may die even due to water change. In that case, changing the water again may worsen the situation.

However, a 10-25% weekly water change in a proper way is recommended to maintain good water quality. Although water change frequency may vary according to tank conditions since every tank is different.

Bearded man changing water
Bearded man changing water

Irrespective of the situation for a water change grab a siphon, de-chlorinator, thermometer, bacteria starter, and a test kit to perform the water change.

If you want to know more details you can read my article on how to change water without killing any fish.

How to Sterilize Fish Tank & Equipment After Fish Died?

So, if your fish tank suffers from a severe outbreak, you have no choice but to sanitize the contaminated tank. In the below section, I’ll describe in detail how you should do that.

man sterilizing the tank.
Man sterilizing the tank.

Method-1: Run the tank Without Fish

Transfer all the fish to another tank, for example, a hospital tank. However, probably none of the fish will survive if the contamination is severe.

Now run the tank without fish for a couple of weeks.

Parasites need a host or carrier to survive. Otherwise, they can’t live and spread the disease.

When you are removing all the fish there will be nothing to spread. If fish are absent in the tank, parasites will not last, they will die off.

In this method, all the bacteria will be preserved, and also you’ll get rid of the infection.

After that test the water parameter. If it’s ok transfer fish that have survived in this period.

Method-2: use Vinegar Water Solution

A vinegar water mix is a good method to sanitize your fish tank. Use a 1:1 solution of vinegar water to clean the tank and associated equipment.

Soak all the accessories and the tank itself for several hours. After that, rinse everything with fresh water, and wipe out the residual vinegar.

Vinegar water

Air-dry your fish tank, and equipment before using it again. If you want to use a stronger solution prepare a 2:1 vinegar water solution.

But, does vinegar disinfect the aquarium? Vinegar gains a boost in its disinfecting prowess if you add some salt to it. In conjunction, distilled vinegar, and salt work pretty well to kill off germs.

Method-3: Use Bleach Solution

To disinfect a contaminated fish tank with deadly aquarium diseases cleaning with a bleach solution is the most practiced and effective method.

Remove all of the decors, substrate, and rocks from the tank, and fill up with bleach solution. You should use household bleach that contains 5%–9% sodium hypochlorite.

Bleach (5%–9% sodium hypochlorite)

The guideline for preparing a bleach solution for cleaning and disinfection according to CDC is to use 5 tablespoons of bleach per gallon of water.

Now clean everything where pathogens can be present with bleach solution. After that rinse twice with clean water, and let dry completely for the next two weeks.

The reason is that a trace amount of bleach present can be deadly for the fish.

So, whether you use vinegar water or bleach don’t forget to cycle the tank, and use water conditioners to condition the water

The most common water conditioners for fish tanks are in the below table:

Name Best For Dose Price Where To Buy
Seachem Prime Removes chlorine & Chloramine. Detoxify ammonia, nitrite, and heavy metals. 5 ml for 50 gallons 20$ for 500 ml Available On Amazon
Seachem Safe It’s a dry version of Shechem Prime, and even more concentrated 1.25 g per 300 gallons 12$ for 250 g Available On Amazon
API stress Coat Removes chlorine, and chloramine and detoxifies heavy metals. Reduces stress on fish, and replaces slime coat on fish. 15 ml per 30 gallons 7.8$ for 16 oz Available On Amazon

How do you completely sanitize a fish tank?

To completely sanitize the fish tank, cleaning with the bleach solution is the most practical way. It is kind of breaking the whole tank setup, and cleaning everything possible.

You need to clean all the fishkeeping accessories including the water changer hose, filter, heater, fishnet, tubing, etc.

Also, you need to cycle the fish tank again to bring fish in there. So, it’s a kind of starting a new tank after cleaning with bleach.

My fish died From Ich, How to Clean Tank?

If the fish died from an internal parasite or fungal disease such as Ich you can use the heat and salt method to sterilize the tank. Although Ich is difficult to treat, it is worth trying to recover.

If it still persists, you can take any of the methods described above to clean or sanitize the tank.

Fish with White Spot Disease (Ich)
Fish with White Spot Disease (Ich)

Heat & Salt Method

  • Increase the tank temperature slowly (over the course of 2 days) to 86°F. keep it there for 10 days.
  • Add some aquarium salt; one tablespoon of salt per 5 gallons of water.
  • Now change the water to 15-25% every two days.
  • After that gradually reduce the temperature.

This method works really well to kill any parasite, worms, and fungus infection.

I have a separate article on treating Ich in fish. I encourage you to read that if you want to know other effective ways to treat ich.

How to Sanitize Fish Tank After Disease Without Bleach?

If you’re not comfortable using a bleach solution to disinfect the fish tank you can use vinegar instead.

Prepare vinegar water solution, and add some salt to augment its disinfecting power.

After that thoroughly clean every part of your fish tank set up. Don’t forget to rinse well before adding water for the fish to keep in.

Bottom Line

You don’t have to sterilize the fish tank every time you have a dead fish. In fact, you only need to sterilize the tank if you have a severe disease outbreak.

Sterilizing the tank is like starting the fish tank from the beginning. So, wait and try to provide proper medication to control the damage, and you should consider sterilizing as the last option.

Sujit Modak

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