Why Do My Pet Fish Keep Dying?

Fish deaths can be traumatic, especially when you don’t know what you’ve done wrong. Here’s how to solve the problem.

May 18, 2024By Kenny Jarvis
why do my pet fish keep dying

Getting any new pet is an exciting and rewarding experience. However, it can also come with challenges that you didn’t even consider. Sadly, many fish owners can find themselves in a situation where their fish keep dying without a clear reason.

Understanding why your fish are dying is crucial for being able to create a healthy and stable environment for them. Thankfully, we’re here to help. Here are some common reasons behind fish deaths and how to prevent them from happening again.

Establishing a Nitrogen Cycle

new empty tank
Source: The 2HR Aquarist

You should never place fish into a brand-new tank. The tank may look pristine and clean, but this is a part of the problem. It lacks the required biological balance for fish to thrive. This is why you need to have an established nitrogen cycle.

Bacteria in your tank will help to break down ammonia into nitrite and eventually into nitrate. That is still harmful to fish but can easily be controlled with water changes. In a new tank, these bacteria have yet to form which can mean ammonia and nitrite can quickly spike, poisoning your fish.

Due to this, you want to cycle your tank fully before adding fish. This means adding a source of ammonia to the tank and giving time for bacteria to colonize your filter and substrate. How long this takes is a hotly debated topic, but it's at least two weeks, ideally up to eight weeks.

When you do add fish, it’s best to do it with a hardy species and keep an eye out on your water parameters. If you’ve recently purchased a new tank, then this could be why. Avoid buying any new fish for a few weeks to allow your tank to cycle properly for a well-balanced aquarium.

Water Quality

cloudy fish tank
Source: Practical Fish Keeping

If you do have an established fish tank, then the next most common culprit for fish deaths is poor water quality. There are a couple of reasons why fish need water changes, and we've already looked at one.

Ammonia, nitrites, and nitrates are byproducts of fish waste and uneaten food. There is only so much your bacteria and filter can process until they ultimately become toxic. One of the easiest ways to keep a healthy aquarium is with testing strips, which allow you to keep an eye on your water quality.

The other reason fish need fresh water is oxygen. Like us, they need oxygen to breathe, and there is only a certain amount of it trapped in water. Having something to agitate the surface (commonly your filter) can help the water absorb oxygen from the air. Stagnant water struggles to absorb any fresh oxygen.

Therefore, your water changes have a double benefit. They remove waste and introduce oxygen. If your fish are dying then ensure the filter is working and right for the tank, do frequent water changes, and have something to agitate the surface water.

Overcrowding in Your Tank

overcrowded fish tank
Source: YouTube

Overcrowding is another common mistake that many people make. An overcrowded tank can lead to increased competition for resources such as oxygen and food. This leads to elevated stress and a rapid buildup of waste products.

Added to these issues, fish are much more likely to get territorial. Even fish that are usually quite friendly can start to act aggressively to their tanks mates if there is limited space. To avoid overcrowding, make sure you are following stocking guidelines.

Many people have their own opinions on this, but the general rule is that you shouldn't have more than one inch of fish for every gallon of water. This should only be loosely followed as some fish, such as goldfish, have a higher bio load than others even though they are one of the best fish breeds for first-time owners.

Poor Tank Setup

oversized filter
Source: Reddit

You need to setup your tank effectively for the fish that you put in it. When buying your filter, heater, decorations, lights, and anything else, make sure they are suitable for your tank. For example, a filter that is too big can exhaust small fish and cause them to get trapped.

A common error is also not having a filter that is big enough, leading to insufficient waste removal. You also want to keep your tank at the right temperature, with a heater almost certainly required if you want to keep tropical fish.

Other mistakes can be made, such as using cheap, sharp artificial plants that can be deadly from small fish trying to swim through them. If your fish keep dying, double-check the tank is suitable for them.

Disease and Parasites in Your Tank

fish disease
Source: The Spruce Pets

Diseases can be heartbreaking for fish owners. It seems as though no matter how hard you try, sometimes your fish get sick. There are a wide range of fish diseases but they usually come with obvious symptoms that tell you something is wrong.

Many diseases can be cured with medication. A common cause is introducing new fish or plants to your tank, so if you did this before you started dying, they may have been carrying a disease. This is why it’s always important to quarantine a fish before adding them to a tank.

Stressed Fish

bad tank setup
Source: Reddit

Stress is probably the most underrated aspect of fish health. Stress lowers their immune system and makes them much more liable to illness and potentially death. A stressed fish isn't going to survive for very long in your tank.

The frustrating things is there are many things that can cause fish to be stressed. A lot of these can be caused by things we’ve already looked at such as poor water parameters, bad water quality, and overcrowding.

There can also be other factors, such as temperature fluctuations, poor diet, aggressive tankmates, and a lack of hiding places. Make sure you’re providing a safe, stable, and consistent environment for your fish.

Inadequate Acclimation

adding fish to tank
Source: Sea Tech H2O

We mentioned temperature fluctuations being an issue, and there are two times when this can be an issue. The first is during water changes, as you want to make sure that the water you're adding is at the same temperature as the tank.

The second is during acclimatization when you're adding new fish to the tank. It's almost certain that the water in the bag will be different to the aquarium, and due to this you need to float the sealed bag in the water for around 15-20 minutes.

After this, make some small cuts or add some aquarium water to the bag. This will help to reduce stress and allow them to adjust gradually. After around 30-45 minutes, you can then remove the bag and allow them to swim free.

Kenny Jarvis
By Kenny Jarvis

Kenny is a passionate animal lover who finds joy in the diverse world of pets. He frequently embarks on zoo adventures with his children, immersing himself in the wonders of nature. At home, Kenny tends to a much-loved aquarium, nurturing a thriving underwater ecosystem. Through his passion for writing, he aims to help fellow animal lovers create happy and healthy environments for their pets.