How to Sterilize Fish Tank after Fish Died

How to Sterilize Fish Tank after Fish Died

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So, it happened. Bubbles, your pet fish, passed away. You found him this morning, belly up in the tank floating lifelessly. But, you don’t how long he’s been this way or what occurred that caused his passing.

Regardless, you have to dispose of the fish and make sure the tank conditions are okay for the rest of your aquarium. Here’s how to clean and sterilize the tank after Bubbles dies.

Remove the Fish

The very first thing you should do is removing the dead fish from the tank the moment you know it died. But, before you do that, use a stick or some other gentle implement to ensure they’re not playing dead. Some fish do that to avoid bullies and predators.

If there’s a cloudy film around the fish, this is a sure sign of death. This is because of the developing bacterial bloom responsible for producing the copious amounts of ammonia to help with decomposition.

When you’re certain it’s not moving or responding, then use your fishnet to remove Bubbles. Put him in a container or dish and off to the side for now. Don’t dispose of him yet. If you used your hands to do this, be sure to wash them well with soap and warm water. Follow this with a dab of hand sanitizer.

Check Water Parameters

The next step is to check the water parameters and make sure everything is still within normal levels. Of course, this is going to depend on how large your tank is. On average, a 30-gallon aquarium will take an entire day before it pollutes.

If there’s a horrid odor emitting from the aquarium, ammonia is plentiful. Although a good idea to measure the water anyway, assume you have a tainted tank. Begin taking steps to clean and disinfect it so that nothing else will suffer.
With a good liquid test kit or an API Master Test Kit, measure the water to see if the readings are as follows:

  • Nitrite – zero
  • Ammonia – zero
  • Nitrate – at least 10 ppm but no more than 20 ppm

Any levels beyond these may damage other fish and plants residing in the tank. For instance, even 0.5 ppm of ammonia can hurt a saltwater fish’s gills. Too little nitrate can be harmful for freshwater fish.

Testing Water

Perform two tests. The first time, measure the water from the area you removed the fish. The second reading should come from the surrounding water in the tank. This gives you an estimate of how long Bubbles left this world.

If ammonia is off the charts in other areas of the tank, the fish has rotted for some time. However, if it’s only in the immediate vicinity, then the passing was recent.

Changing Water

For a high reading concentrated in the area from which you removed the fish, try to remove that water and simply add a little bit more if necessary. Wait an hour or two and do another reading of the tank in the same way. If everything is within normal ranges, your tank is good to go.

But, if the fish died from illness, this can pose a health risk to the rest of the inhabitants who may have consumed it. Depending on the readings from your tests, you might only have to change a portion of the water. In the event your water parameters are totally off, you have to sterilize the tank right away.

This will especially be true if you see other fish dying or suffering as a result of the high levels of ammonia. The goal is to reach ammonia levels that read .025 ppm or less as quickly as possible without stressing out the remaining fish.

Methods of Changing Water

For 20% to 50%, you can remove the water in 10% intervals once an hour until you reach the desired range of ammonia. If there’s 50% or more ammonia in the tank, you’ll have to change the water 50% or completely clean out the tank.

Now, if you have a tank with something like an anemone or other species sensitive to water changes, you will not be able to move everything out completely. This means, you’re going to have to work quickly, steady and with patience to ensure nothing adverse occurs.

A complete water change is not advisable so as to not stress other fish in the tank. But, it could be necessary if your ammonia levels are above 80% or if all the level readings are out of whack. So, you want to get everything in the tank into another clean tank with fresh and appropriate parameters.

Regardless of the percentage levels, it’s wise to use a siphon designed for fast and quick water changes.

Sterilize and Refresh the Tank


If you’re doing a total water change, clean out the tank well with hot water and plant-based dish soap. Rinse the aquarium thoroughly; ensuring all soap is gone from the tank. Then, take one part bleach and six parts warm water in the aquarium along with any other mechanical parts that might need it, like filters and pumps.

Allow this to soak for about 20 to 30 minutes. Rinse everything well with warm water and then again with cold water. Make sure there’s no bleach smell at all. When in doubt, continue rinsing.

Then, fill the tank with fresh water according to the appropriate parameters. But, keep in mind you’ll want to also add a large portion of the water from the transfer tank. This will help to prevent further shock and stress of the surviving fish.

Retest the Water

But, before returning everything back to the original tank, make sure the water parameters match that of the transfer tank. Once temperature and chemistry is within a good range, via another reading from your water test kit, add the fish back in.

Bacteria Booster

Next, regardless of whole or partial water changing, pour in a beneficial bacteria booster. These will convert remaining ammonia into beneficial substances within the aquarium and handle any residual pollution.

The brand of bacteria booster you use will depend on the type of water in your aquarium and the needs of your other fish. So, you’ll have to do some research, ask questions and shop around. Aside from initial considerations, make sure you get a trusted brand and don’t hesitate to invest in higher-end stuff.

Saying Goodbye to Bubbles

Once the tank returns to a stable, normal state, it’s time to say goodbye to Bubbles. Avoid flushing him down the toilet if he was sick or had parasites. This is because it can present a greater issue for the local ecosystem.

Wrap him in something airtight and either put him in the garbage outside or bury him about two to three feet deep in the yard. You want to make sure no wild animals will smell him and try to dig him up.

Related questions:

Cause of Death

Once all the fish and plants are safe, it’s time to inspect Bubbles. Here’s a list of some of the most common causes of death:

  • Illness or Disease
  • Injury from other fish  
  • Injury from sharp decorations
  • Old age
  • Polluted water
  • Stress
  • Thermal shock from water temperature shifts
  • Using chlorinated water

Disease or Parasites

If the stomach or abdomen is large and bloated, this suggests disease. If the body or fins have bite marks or rigidness, another fish could’ve bullied Bubbles to death. If there are white spots or patches of discoloration that weren’t there before, it’s a sign of a parasite, called ich or ick.

Look at the other fish to see if you see similar spots on them. If this is the case, you can get a formaldehyde-based medicine and increase the tank’s temperature. This should clear up the problem in no time.

When you suspect disease or parasites, keep an eye on the other fish, especially when there’s evidence of other fish nibbling on the corpse. Also, check the difference in water temperature between the daytime and nighttime. Any small shift may have been stressful to Bubbles and perhaps what caused him to die.

Other Causes

It could be that the tank was too small for Bubbles from the start. Or, maybe he was a schooling fish and didn’t have enough buddies to keep him happy. Another consideration could be that he was old and it was his time to travel to the great beyond.

Once you’ve been able to inspect the body and determine the cause of death, put Bubbles in a plastic box or bag and place him in the freezer. Don’t flush him down the toilet, yet.

See also:
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