How To Sterilize Your Fish Tank After Fish Died

It’s never a great feeling to find that one of your fish has died, and depending on what caused the death, you may need to sterilize the tank. If you find a fish floating at the surface or lying on the gravel, there are ways to deal with cleaning the tank to prevent further deaths.

How to sterilize your fish tank after fish died starts by removing all decorations and equipment and throwing away anything that cannot be sterilized. Wash the tank and equipment in a bleach solution, and rinse thoroughly. And, you will have to cycle the sterilized tank as though it were a new tank before adding fish. Keep reading for more details below.

Should You Clean Your Fish Tank After a Fish Dies?

Cleaning the tank seems the obvious solution to stop the spread of the disease that killed your fish, but you may not need to take as drastic an approach. Sterilizing a tank is a big process, and resetting an aquarium for healthy fish takes time.

If you see a fish floating on the surface of your tank or one that has sunk to the bottom, you need to remove the fish. If no other fish or aquatic pets are living in the tank, you can begin the process of cleaning and sterilizing it.

However, if you still have other fish in the tank, you want to minimize their disturbance. Most fish die because of poor tank hygiene, and even if you completely break down the tank and sanitize it, your new fish will also die if the original hygiene issues persist.

  • Check your filtration system – sponge filters must be cleaned every 2-3 weeks to maintain water cleanliness; canister filters should be cleaned every 1-3 months.
  • Test the water quality with testing kits. Your fish may have died from an ammonia spike or other issues with water quality. Ammonia test kits use a different testing method, so they are sold separately from the usual 6-in-1 test strips.
  • Overfeeding – leads to an increase in bacteria colonies and algae outbreaks.
  • Disease. You might be able to see signs of disease on a fish. Disease is usually caused by improper tank maintenance and dirty water.

How To Sterilize Your Fish Tank After Fish Died

Exactly how to sterilize your fish tank after your fish died depends on the cause of death. After you remove a dead fish, check what might have caused the death. If the death was caused by something you can fix, such as water temperature, you will not have to sanitize the tank.

If your fish died because of disease, this requires a little more analysis.

Some diseases, such as fin rot, are caused by poor water quality and overcrowded conditions, while severe parasite infestations, like velvet, are issues that might require tank breakdown and sterilization.

If it’s necessary to clean out and sterilize a tank due to disease, you must let it cycle for 6-8 weeks after setting it up before adding new fish. The sterilization is only the first part; you must maintain good water quality to prevent your new fish from dying.

As you can see in the steps below, sterilizing your tank is quite a process. Once your tank has been thoroughly cleaned, sterilized, and dried, you can begin to set it up as though it were a new tank. It will take around two months before your aquarium has cycled to the appropriate ammonia and nitrate levels.

Sterilization Steps for Cleaning Your Fish Tank

Disposal of Tainted Items
  • If there are still living plants in the tank, dispose of them.
  • Remove all the tank’s gravel, rocks, filters, and other items.
  • Throw away anything that will be difficult to clean thoroughly, such as gravel and filter media.
Disinfecting Wash
  • Wash all the filters, intake tubes, and large rocks or artificial plants in very hot water, to which you can add some bleach in a solution of 1 part bleach to so parts water. You can leave them to soak for a day.
  • Clean the tank with a 1 to 20-part bleach and water solution. If you do not want to use bleach, a 1-10-part vinegar and water solution is also an option.
  • You may prefer to sterilize artificial silk plants by thorough cleaning, then leaving them in the sun.
  • Do not soak porous materials like wood in your bleach water, as the bleach will be absorbed.
Rinse & Dry
  • Rinse out your tank and all tank filters and décor to remove any traces of bleach or chemicals. Do this several times to ensure all the bleach is gone.
  • You can dry out your tank and leave it for a few weeks, as this will kill any bacteria or parasites.

Safety Precautions When Sterilizing a Fish Tank

If you use bleach to sterilize your tank, remember that it is a dangerous chemical, and you should exercise caution.

  • If there are other tanks or fish in the room, ensure they are covered; no bleach solution can enter them.
  • Preferably, sterilize your tank and equipment in another room.
  • Wear rubber gloves when handling bleach and washing out the tank, decorations, and equipment
  • If treating a tank with potassium permanganate, you should wear gloves and protective eyewear. Remove your filter when treating the tank with PP, as it can damage the helpful bacteria in the biological filtration system.
  • Wear protective gloves and eyewear if treating a tank with a toxic poison like formalin. Formalin can create a toxic gas, so use only as advised and in a ventilated area.

Should I Change The Water After a Fish Dies?

If a parasite or disease did not cause the fish’s death, you might not need to empty and sterilize the tank. If there are still other fish in your tank, you don’t want to kill or disturb them, so how do you clean the tank in a situation like this?

You can take steps to get on top of water quality immediately.

  • Remove any dead fish or rotting debris.
  • Check the nitrate and ammonia levels in your tank using a test kit. You should do this daily if possible.
  • Clean the glass to remove algae.
  • Siphon or vacuum your gravel to remove any fish food or waste that may have accumulated in the gravel and created a breeding ground for bacteria. As you clean the gravel, you will also empty dirty water.
  • Replace the dirty water with fresh water treated with a tap water conditioner to make it safe for aquariums. You can also use a UV sterilizer to help decrease unwanted elements such as bacteria and algae.
  • When replacing tank water, you want to disturb the fish as little as possible. Change approximately 15-20% of the water in the tank every week. Smaller tanks require smaller, more frequent changes, while larger tanks tend to have more stable conditions.
  • If you are certain there is a parasite infection, you can treat the tank using a potassium permanganate solution. You must read the instructions thoroughly so as not to cause further distress or death to your fish.
How To Sterilize Your Fish Tank After Fish Died

Keep Your Tank Healthy to Avoid Fish Death

Keeping your tank healthy should avoid fish death from diseases and parasites. You can ensure filter material is cleaned and replaced regularly, avoiding algae and debris build-up and maintaining healthy water conditions.

Fish can also die from stress and bullying, so ensure you have the right mix of species in your tank and that it is not overcrowded. When introducing new fish to the tank, acclimatize them slowly before releasing them into the tank.

Some fish owners have a quarantine tank to acclimate their fish before adding them to the man tank. This way, they can check that the new fish is not bringing in parasites or diseases that could wipe out the other fish.

Also, consider quarantining or disinfecting new plants before adding them to the tank to prevent the spread of unwanted parasites or snails.


The best steps for how to sterilize your fish tank after fish died include checking your filtration system, sterilizing your tank and its components, and possibly changing the water. Keeping a clean tank and making sure your fish are not stressed, are simple ways to prevent your fish from dying in the first place.

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