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1.6. Aquarium Fast Fish Deaths

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Author : David Bogert

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Time To Read :
9 minutes
Difficulty : Level 4

Excerpt :

There are a lot of things which kill aquarium fish overnight. Chief among these are super-chlorination and bacterial infections.

There are some things which kill lots of fish rapidly with no symptoms:

  • If the fish died just after a water change:
    • With chlorinated water and a large water change, a chlorine pulse can be to blame.
    • With well water and a large water change, low oxygen and/or high carbon dioxide can be to blame.
  • With no water change a bacterial infection is the most likely culprit.
  • An overpowered heater can malfunction, overheat the tank, then return to normal operation without the hobbyist being any the wiser.
  • If the aquarium is heavily stocked and a power outage occurs for more than a few hours, the fish can die from asphyxiation. Like a heater malfunction, the power can come back on and the hobbyists can be unaware that a power outage even occurred.

And there are other possibilities. Note the sudden death of several fish typically has nothing to do with ammonia or nitrite. Ammonia and nitrite are slow poisons, not fast poisons.

“Bad water parameters” (ammonia, nitrite, nitrate) cannot rapidly kill fish. Neither can “stability” problems, “poisonous gas” or electrocution rapidly kill fish. It just doesn’t happen.

Trigonostigma heteromorpha Harlequin Rasbora
Trigonostigma heteromorpha – Harlequin Rasbora

Most of the Possibilities

These are most of the possible reasons for the fast deaths of many fish in an aquarium:


Municipalities supplying water to homes sometime do what is called a “chlorine pulse”. Sometimes water suppliers put up to 4 ppm chlorine in the water. The normal dose of the conditioner will neutralize 1 to 2 ppm. That leaves 2 to 3 ppm to kill the fish. I once lost fifty large Malawi’s to a chlorine pulse.

Penguin Tetra
Penguin Tetra

The topic of a chlorine pulse is explained at length in this link:

5.5. Chlorine and Chloramine

And sometimes there is a type of chlorine kill that we feel bad about. Even the best of us make a common mistake. We simply forget to add a dechlorinator during a water change. And we get a tank of dead fish. The excellent YouTuber Tazawa did this. Killed a tank of fish. Tazawa is a credit to the hobby so we felt really bad for him.

Forget to add a dechlorinator during a water change
Black Molly
Black Molly

Well Water

IF AND ONLY IF one has a well and one does not have an aerating tank on that well, one can lose fish by a mechanism that mimics chlorine poisoning. Well water is typically very low in oxygen. If one does a 50% water change this can drop the oxygen levels by 50%. Many fish die quite rapidly at these levels. In a well-aerated aquarium the levels will come up rapidly (in a few hours) but many fish can be dead before that point is reached. So either the water needs to be aged for 24 hours or one should only do a maximum of 25% water change.

This well water can also have large amounts of carbon dioxide in it which adds to the low oxygen effects.

Rasbora trilineata Scissortail Rasbora
Rasbora trilineata – Scissortail Rasbora

Bacterial Disease

Another common rapid killer of fish is bacterial diseases, largely columnaris or Aeromonas. These bacteria can rapidly kill fish with no outward symptoms apparent.

A newly set up aquarium that has only been running for a few days or even a few weeks will have a lot of bacteria in the water column. Add very stressed new fish from the fish store and they often die overnight, overwhelmed by the combination of stress and bacteria. This is extremely common and invariably gets blamed on ammonia poisoning (i.e. “new tank syndrome”)

Bacterial diseases can arise spontaneously in an aquarium without the addition of any new fish.

Xiphophorus maculatus Red Coral Platy
Xiphophorus maculatus – Red Coral Platy

Sometimes hobbyists do not do proper quarantine and get an outbreak of bacterial disease from new fish.

A very common thing is for a lot of fish to die and some to have white spots on them. The hobbyist then comes on social media and says their fish died from ich. Invariably when pictures are obtained it isn’t ich but rather a combination of columnaris bacterial infection followed by secondary epistylis. Epistylis looks like ich.

Treating bacterial diseases is covered in this link:

10.3. Bacterial Diseases

Epistylis is covered in this link:

10.2.4. Epistylis

Redeye Tetra
Redeye Tetra


Fish from a fish store has been through a lot when they are poured into a new tank. They’ve been chased, netted, bagged transported, and poured into a new tank. They often have just recently been subjected to rapidly enlarged swim bladder by being flown to the local fish store from Asia. Fish don’t do well with this handling. It often kills them rather rapidly. Note that it is the handling that kills them, not a difference in water parameters or temperature.

Sailfin Mollies
Sailfin Mollies

VERY Hot or Cold water

It is surprisingly common to have a single large heater and have it stick on, literally parboiling the fish. Using either two heaters or a heater AND a controller is important in any aquarium. It is also surprisingly common to have aquarium thermometers or controllers go “bad” and give completely erroneous readings. 94°F (34°C) or 60°F (16°C) can kill fish.

And sometimes a hobbyist uses cold tap water to change the water in the aquarium with fish that can’t take water below 60 to 65 °F (16°C to 18°C). Or they let an aquarium drop in temperature below 60 to 65 °F by putting the aquarium next to a cold window. And sometimes a hobbyist can have a power outage in a cold room and have the temperature drop below 60 to 65 °F.

Xiphophorus maculatus High Fin Platy
Xiphophorus maculatus – High Fin Platy

Dead Fish

There is a common scenario with dead fish. Someone sets up a tank and puts in some fish without cycling the tank. When the fish are put in the aquarium one of the fish dies of shock. The death isn’t noticed. Because the fish are skittish and shocky, they don’t eat the dead fish. The dead fish decomposes. The decomposition products include a bunch of organic toxins and bacteria which kill most of the other fish in the tank overnight.

Note that the decaying fish body can also spike the ammonia somewhat, something which will only contribute to the deaths, not cause them directly. And the new hobbyist comes on social media with their story. Then everyone on social media tells them ammonia killed their fish because the tank wasn’t cycled. This type of mass fish death rarely happens in well-filtered long-established aquariums.

Sawbwa replendens Rummynose Rasbora
Sawbwa replendens – Rummynose Rasbora

Lack of Aeration

It can be deadly to a heavily stocked fish aquarium to have a power outage as short as three hours. I lost most of the fish in three heavily stocked Malawi Tanks when the power went out. I now have a generator. Note that one can have a power outage for some time, kill some fish, then have the power come back on. If one is asleep or at work one might not know about the power outage.

Sometimes the hobbyist creates the aeration problem. Ben Ochart was using powerheads aimed at the surface in one aquarium. He turned the powerheads off overnight to “give the fish a rest”. Most of his fish died from lack of aeration. Ben is a credit to the hobby so we felt bad for him.

Powerheads in a aquarium
Serpae Tetra
Serpae Tetra


Some medications are a problem. Even slight overdoses can kill. Blackwater fish are especially vulnerable.

  • Formalin (formaldehyde) that is found in ich medications can kill if overdosed
  • Chloramines (Lifeguard All-In-One, 1-chloro-2,2, 5,5-tetramethyl-4-imidazolidionone) can kill
  • Seachem Paraguard is gluteraldehyde and can and has killed fish even at the recommended dosage.
  • Copper medications can kill if the water is soft (GH<3).
  • All “algae killers” are poisonous to fish and have killed whole tanks at the recommended dosages.
  • “Liquid carbon” (gluteraldehyde) is poisonous if overdosed.
Xiphophorus maculatus Platy
Xiphophorus maculatus – Platy


If a hobbyist uses chemicals like floor strippers or insecticides in the aquarium room, the fish can be killed very rapidly. Most household insecticides are now pyrethroids, which are quite toxic to fish. And don’t even think of leaving fish in a house being “tented”. The gas used in tenting will penetrate the best-wrapped aquarium in the world.

Silvertip Tetra
Silvertip Tetra

Things Which Don’t Rapidly Kill

What is noteworthy is what is not on this list. “Bad water parameters” cannot rapidly kill fish. Neither can “new tank syndrome”, “stability problems”, poisonous gas, or electrocution rapidly kill fish. It just doesn’t happen. Each of these myths is examined below:

“Bad Water Parameters”, Ammonia, Nitrite and Nitrate too high

Ammonia and nitrite are NOT rapid killers of fish in an established tank, contrary to the many myths. The acute toxicity of various compounds in the aquarium is as follows:

pH of the waterAPI Ammonia test in ppmAPI nitrite test in ppm
Nitrate alarm level is greater than 80 ppm.
Nitrate toxic level is greater than 440 ppm.

To read the numbers for ammonia and nitrite simply determine the pH of the aquarium and find that pH on the left column. Then read across the levels that the API test will show. Start doing water changes at the yellow alert level.

The numbers for the toxicity of ammonia, nitrite, and nitrate will come as a shock to most experienced hobbyists. But the numbers are supported by a lot of research by universities. They are solid. It is rare for these numbers to be exceeded in an established aquarium. Here is a link to many articles on the subject:

5. Ammonia, Nitrite, Nitrate, and Chlorine

Strawberry Tiger Barb
Strawberry Tiger Barb

“New Tank Syndrome”

IF a hobbyist sets up a new tank and IF a hobbyist adds a huge amount of fish to that new tank and IF a hobbyist feeds a huge amount of food, then and only then will the fish die from bacterial toxins, low oxygen levels, and bacterial infections. Note it generally is NOT the ammonia or nitrite that kills the fish. This is much less a problem than social media makes of it. More about it at this link:

2.7. New Tank Syndrome

Endler Guppy
Endler Guppy

“Stability” Problems

Contrary to popular myth, rapid changes in any water parameters (temperature, pH, KH, GH, etc.) do not kill fish. In their native waters fish are constantly hit with sizable variations in water parameters and they do just fine. Here is a link to a full analysis:

4.8. Stability is not Important

Dalmation Molly
Dalmation Molly

“Poisonous Gas”

Poisonous gas (i.e. hydrogen sulfide) categorically does not form in a substrate. Can’t happen, never happened, and won’t ever happen. This myth is delved into in more depth in this link:

14.2.4. Anaerobic Myth

Trichopodus leerii Pearl Leeri Gourami
Trichopodus leerii – Pearl Leeri Gourami


One can probably count the number of aquarium fish that have died from electrocution worldwide in a year on one hand. It is theoretically possible to electrocute fish. But the combination of events that would lead to such an occurrence is virtually impossible in aquariums. This is analyzed further in this link:

14.11. Fish Electrocution