Top 13 Most Likely Reasons Why Your Aquarium Fish Died

Why did my fish die?

It is the most frequently asked question about aquarium fish. Each year, people buy millions of aquarium fish. Most of the fish species have a natural lifespan of at least several years. Some of them can live up to a decade or more in the wild. Somehow most of the fish died within weeks after purchase. The average lifespan of aquarium fish after leaving the shop is no more than 3~4 weeks. Many of them died within days, or even within hours. They are the most mistreated pet animal in the world.

For sure, fish are fragile creatures compare to us. It does not mean they died for no reason at all. There is always a cause. Majority fish deaths are due to some beginner mistakes we can identify and avoid. Some of the errors are more common than others.

For example,
A lot of the fish deaths occurred soon after new fish are introduced. Also, a lot of fish deaths took place after a massive water change. Some other fish died after a large feeding. Anyway, the majority of them died in the first month.

In the following, we will discuss the top 13 most common causes of aquarium fish deaths and how to avoid them. They are not necessarily in any order. Some of them are closely related.

1. Fish died in shock because there is a sudden large change in water temperature, PH, hardness, etc. when the fish owner put them in the fish tank.

Since fishes have no body heat, they are sensitive to sudden changes in water temperature. Water PH and water hardness can also affect them if there is a sudden large swing.

When you brought the fish home from a pet shop, or from elsewhere, it is usually in a plastic bag of water. It is most likely the water in the plastic bag is very different from the water in your home aquarium.

“Dump” the fish in the fish tank right away can kill them quite fast. If the fish are in severe shock, they usually die within hours, if not by the next day or two. Most of the quick deaths of fish are due to shock.

Solution: You must “acclimate” the fish slowly to give them time to adapt to the new water.

2. Fish died to untreated tap water

Tap water is perfectly fine for aquarium fish as long as you have used an aquarium water conditioner. Without the aquarium water conditioner, the chlorine and chloramine in the tap water can kill the fish.

Solution: Just buy a bottle of aquarium water conditioner. They are available everywhere. A single bottle can last for a very long time. And it works almost immediately when you add it to the tap water.

3. Tropical fish died without a heater.

Tropical fish are from warm water ranged from 70~80F+ (over 20C). Unless you live in a tropical area, the water In the fish tank can’t be warm enough without an aquarium heater.
Since the majority of the fish in aquarium fish trade are tropical fish, they are more sensitive to the water temperature.

Solution: We must use an aquarium heater in a tropical fish aquarium, or the fish will freeze to death.

4. Fish died to lack of oxygen

Fish needs dissolved oxygen in the water. They will die if the dissolved oxygen in the fish tank runs out. Several things can cause it to happen.
a. The fish tank is too small. Less water means less dissolved air.
b. Overstocked the aquarium with too many fish. The more fish you have, the more oxygen they need.
c. The water temperature is too high. The higher the water temperature, the less dissolved oxygen.
d. There is no air pump.

Solution: Get an aquarium air pump. It can greatly increase the rate of gas exchange between the water and atmosphere.

5. Fish died due to ammonia poisoning because there is no aquarium filter

An aquarium filter is mandatory. It is for biological filtration. Without it, the Aquarium Nitrogen Cycle won’t start. As a result, toxic ammonia will build up and kill the fish.

Fish produce ammonia as a waste product. Their poop along with leftover fish food will rot and produce more ammonia. Ammonia is toxic to fish. The concentration of ammonia must stay at 0ppm (part per million) to be safe for the fish. In a closed system, ammonia can’t go anywhere but to build up.

Solution: Buy an aquarium filter, and have it run 24/7.

6. You have not cycled the new aquarium/fish tank/filter.

A new aquarium filter must be fully cycled to maintain the aquarium nitrogen cycle. It takes a source of ammonia and 4~6 weeks to cycle a new aquarium fully. During which time, ammonia and nitrite will build up. Both will increase to a very high concentration before they decrease. However, most fish can’t last that long. Most if not all fish will die before you can fully cycle the filter in a new aquarium.

Solution: Do fishless cycling before adding any fish. Or get a bottle of live bacteria products such as Tetra SafeStart to jump-start the aquarium nitrogen cycle.

7. Fish died to overfeed

Overfeed fish can cause a digestive problem and kill the fish directly. More food than it is needed can also cause water pollution and create more toxic ammonia and nitrite which will kill the fish too.

Solution: Feed the fish only once a day, with no more food than they can eat in less than a minute. In this case, less is better.

8. Fish died in a fish tank that is too small

A small fish tank has very little water. Too little water means too little-dissolved oxygen which we have discussed earlier. Too little water means more unstable water temperature, hardness, PH. Pollution also builds up more quickly. Some people even keep fish in small containers, bowls, cups, jars. None of which is suitable for fish keeping.

Solution: In general, we do not recommend getting anything smaller than a 5-gallon fish tank. The bigger a fish tank is, the easier to keep the fish alive. The bigger, the better.  You must find a suitable fish tank for your fish.

9. Fish died to unsuitable fish food
Some people feed fish random food items such as rice and bread. It does not work that way. Fish have very different diet requirements. They need mostly easy to digest source of protein and fat. Carbohydrate is useless to most fish species. They can’t sustain themselves by eating rice, bread, or any random human foods.

Another example is, some marketers sold betta fish in a flower vase with a plant. They claim the betta, and the plant will sustain each other. How? Betta might be hungry enough to chew on the plant roots, just like humans in famine might eat grassroots and tree skin. But they will still die of malnutrition. Because betta fish is a carnivore that feeds on live insects in the water.

Solution: Buy a bottle of real fish food. They will last months. With some supplements such as bloodworm and brine shrimp as nutrition treats.

10. Fish died of diseases and parasites

There are hundreds if not thousands of fish parasites and diseases out there. For example, ick, mouth rot, fin rot, gill fluke, etc. just to name a few. Many of them are contagious to the fish. A single sick fish might cause the entire fish stock to die if left untreated. Sometimes people found their whole tank started to die after they have introduced a few new fish to the aquarium.

Solution: Buy healthy fish from a reputable source. If you buy local, pick fish from healthy tanks with no sign of sickness, parasites, and dead fish. Isolate new fish in a quarantine tank for a minimal three weeks if you already have an established aquarium. You might also want to relocate the sick fish to a separated tank if you are not sure it is contagious. Treat the infected fish as soon as possible with proven working fish meds.

11. Fish died after an improper water change.

As mentioned earlier, fish can die to a sudden change in the water. When you do a large water change in the wrong way, it can also cause shock to fish and even kill them.

Some people changed 100% of the water, and some others even took the fish out of the water during a water change. Not waiting for the new water to have the same temperature as the water in the fish tank is another big problem.

Solution: Never change 100% of the water, and never take the fish out during the water change. It is called “partial water change” for a reason. Under normal circumstances, only replace 30~50% of the water once a week. In any case, do not change more than 70~80% of the water at once. Before the water change, we must prepare the new water in advance to allow it to rise to the same temperature as the water in the fish tank. Use an aquarium heater to heat up the freshwater before using it if necessary.

12. Fish died without a water change for too long

Even if you have fully cycled the aquarium filter, the final product of the aquarium nitrogen cycle – nitrate will build up. While nitrate is relatively harmless, too high concentration can affect the fish’s immune system in the long run. It can also crash the water PH.

If you have finally done a partial water change after many weeks or even months, it will restore the water buffer and PH all at once. Fish won’t like the sudden change, and they can get in shock.

Solution: Do a partial water change of 30~50% once a week to keep nitrate in check, and it is also to keep the changes in water more gradual.

13. Fish died because they are unhealthy, to begin with (poor choice of where to buy the fish from is a mistake too, right?)

Sometimes, fish still died when we have done everything right.

Many fish farms have poor conditions for their fish. They overbreed them, feed with cheap fish food. Some of the fish farms use tons of antibiotics in the water to keep fish alive. As a result, these fish are weak. They might die soon after they left the antibiotics filled water even if you do everything right.

In general, most Asian fish farms have a lower quality of aquarium fish. European and American fish farms are better managed.

In other cases, the middle-man keep fish in poor conditions. Places like Walmart and many chain stores have interconnected aquarium system. That is, all of their fish tanks use one central filter system. One fish with parasites can potentially infect all the fish in all tanks.

I have first-hand experience with aquarium fish from different sources. The fish from the better source is naturally more expensive, but they worth the extra cost since they do not drop dead like flies.

Solution: Buy fish from a good source. Make sure you know their origin. Some reputable online vendors (yes, they ship fish) list the source of their fish next to the price. If you buy from a local shop, pay attention to the conditions of the fish.

There are other causes for fish to die in an aquarium. But we have covered the most common ones. Overall, fish should not die too quickly in large numbers around the same time. If it happens, you know there is a problem. We can avoid most fish deaths by preventing the beginner mistakes. Just keep in mind that fish might still die on occasion for unknown reasons if you have done everything right by the book. As long as only one fish died (out of many) at a time in a long time (weeks, months), you should sleep well knowing it is not your fault. Not saying you should skip sleep if you have made a mistake. Just please learn from the mistakes if it happens. We were all beginners once, right? 😀