Why Did Just One Of My Fish Die: Not All Of Them?

Nothing is more heartbreaking than a dead fish for a fishkeeper. Fish can die for many reasons. Probably, you have lost your favorite fish, and searching for why did just one of your fish die?

If only one fish dies it’s mostly because of age, diseases, and unsuitable water parameters for that fish. Fish, like all animals, die at the end of their lifespan. Fish may die if it was affected by a disease. Another issue is inappropriate tank conditions, where fish didn’t get enough oxygen or the temperature, and pH wasn’t in the ideal range for the fish.

This blog post will explain why did just one of your fish die, while others are doing fine. I’ll explain the unexplained death of your fish, and find out what probably went wrong.

Why did only one of my fish die? [Answer]

There are several reasons for fish death. Most often it’s because of the poor water quality and therefore it’s recommended to maintain water quality to an optimum level.

However, you can still experience fish death even if you have tested your water and find everything is ok.

Let’s find out what could be the reasons for dying one of your fish randomly.

Age/End of Lifespan

You can’t stop your fish from aging. Fish grow older and die when it reaches the end of their lifespan. So, age should be your first consideration if only one of your fish has died, and others are healthy.

Although you can increase the lifespan significantly by providing the best possible environment, however, there is a limit.

It’s difficult to understand the age though if you didn’t breed it and simply bought that from a pet store. Fish in the pet store usually is 10-12 months old, calculate fish age considering this.

If it’s because of age, don’t be disappointed. Instead, be happy that your proper care helps the fish to reach its maximum lifespan. If it’s not because of age, see other reasons below.

Low oxygen Level

Low oxygen level is very common if you don’t have an air pump or filter suitable for the tank size, and stocking density. It may occur also if you are failed to maintain or clean your tank routinely. High water temperature and heavily planted tanks with low lighting are other causes of low oxygen situation in your tank.

Check on all the reasons for the lack of oxygen in a fish tank here.

Not all fish can survive in a depleted oxygen situation. Labyrinth fish like betta or catfish are able to live in low oxygen levels because they can suck oxygen from the air outside.

Larger fish requires more oxygen than smaller fish. If a lack of oxygen persists in your tank relatively larger fish may die. So, if you’ve lost the bigger fish in your aquarium, check for the oxygen level.


The disease is another common cause of fish death. However, sometimes it’s difficult to notice beforehand If you’re unfamiliar with fish disease.

Fish become prone to disease when they are stressed. Providing a stress-free environment for your fish is vital to keep your fish happy, healthy, and glowing.

Common diseases for fish death include ich, columnaris, swim bladder diseases, etc. If you’re serious about fishkeeping, I suggest you must learn about at least some common fish diseases and their symptoms.

Wrong Tank Mates

Fish combination

The wrong fish combination is probably a common mistake done by beginners. You can’t simply keep fish together randomly.

Fish that are incompatible shouldn’t be kept together. For example, tank parameters aren’t the same for cold-water fish & tropical fish. Cold-water fish requires 60-75 degrees Fahrenheit, whereas 75-80 degrees Fahrenheit is the ideal range for tropical fish.

Keeping cold water fish in a tank maintaining 75-80 degrees Fahrenheit will be detrimental, and vice versa. pH is also a vital consideration; some prefer slightly alkaline water, some thrive in neutral pH, and others do well under a slightly acidic environment.

Keeping fish in water of inappropriate pH is harmful to the fish, and might even lead to death.

Fish temperament is another crucial criterion. Not all fish get along well. Some fish are extremely territorial, and aggressive while others are peaceful in nature.

Cichlids, Betta, and Gourami have a fighting tendency. They may fight over food and can attack the other fish in their territory which can result in fish death. Before mixing fish in your aquarium, please do some research if they are compatible, and can co-habit peacefully.

Small fish can also be eaten by large fish like Oscar. So, never put a fish small enough that can be eaten by other big fish in the tank. You should be careful in this aspect as well.

Improper Tank Size for The Fish

Too small a tank for a large active fish that loves to have open space for swimming may get stressed out. This can eventually lead to death. As I said earlier, a stressed fish is vulnerable to diseases, and improper tank size can be the underlying cause.

It’s also a beginner’s mistake, they purchase the tank not considering the adult fish size. As time passes, when the fish grows it becomes gradually unsuitable for fish. So, before buying a tank make up your mind about what fish are you going to keep, and consider their grown-up length.

The Fish Was New To The Tank

If you bring a new fish into the established tank it’s advised to properly acclimatize the fish before putting it in. Also adding the fish directly to the tank without quarantining may bring parasites or diseases from the pet store you bought.

Keep the fish separated in a quarantine tank, and make sure to acclimatize before transferring it to the main tank.

Quarantine Tank

What should I do if one fish dies?

Definitely, you don’t want any more fish death in your aquarium. Therefore, you should act fast to prevent further damage.

If one fish died the best action is to remove the fish promptly. If the fish died from a disease it can spread to other fish. Also, the fish body will start to decompose in the bacteria-rich tank water which will pollute the tank and can cause harm to the other healthy fish.

After removing the dead fish, you should perform a water change (25%). Then test the water to check if the water parameters are all healthy.

More Resources:

Wrapping Up

I have been keeping fish for years and I had experienced the random death of one of my fish a few times. In my case, it happened mostly when I brought new sets of fish for my tank. One or two fish from the new batch has died.

However, once I started using the quarantine tank, the problem was resolved. It never happened again. New fish died in the quarantine tank (it was affected by the disease, or couldn’t take the journey fatigue), but never in the main tank.

Other times, I experienced random fish death only because of old age. So, for your case try to understand the reason for death, and then take action so that it can’t happen again.

Sujit Modak

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