Skip to content
Previous article
Now Reading:
Why are My Pond Fish Dying?
Next article

Why are My Pond Fish Dying?

Every pond owner's worst nightmare is walking out to their pond and seeing dead fish. Your fish can be seemingly healthy one day and then dead the next, leaving you to wonder why your pond fish are dying. Fish kills can happen overnight due to turnovers or poor water conditions. Learn more about the common causes of fish kills in ponds so you can prevent them and keep your fish happy and healthy. 


In the spring and summer, as the pond gets warmer, the deepest part is often the coolest and has the least amount of oxygen. If you don't have a filtration system and water features or an aerator that properly keeps the pond circulating, the water can stratify. This means that the water separates itself into layers by level of oxygen and temperature. 

When choosing a pond pump, you want to make sure that it's strong enough to turn the pond's total water volume at least once every hour. If you don't have a strong enough pump, only the water at the pond's surface circulates and is able to oxygenate. 

When this happens, the pond stratifies. The water at the surface is warmest from the sunlight and has the most oxygen. The water at the deepest part of the pond has the least amount of oxygen and is coldest. Depending on how little circulation you have and how deep your pond is, the bottom of the pond may be devoid of oxygen altogether. 

The good bacteria that you want in your pond to break down organic material and waste need oxygen to colonize and work. The harmful bacteria don't need oxygen. When the bottom of the pond loses oxygen, the harmful bacteria take over and cause water quality issues like bad smells and algae blooms, which can decrease oxygen levels even more.

Turnover is when the water from the surface and bottom of the pond quickly mix. A storm can cause turnover as the rain pushes water from the surface into the deeper parts of the pond. During turnover the overall oxygen levels of the pond can drop, causing a fish kill. 


Overcooling is similar to turnover, but the cause of the fish kill is not oxygen levels but temperature. In colder months, the deepest part of the pond is actually the warmest. This is where fish overwinter. They need less oxygen as they go into a near hibernation. The surface of the pond is the coldest, which is why it forms ice first. 

Once the water temperature drops to 40ºF, you need to bring in your pond equipment. If you leave your water features running, they can cause your pond to turn over. This type of turnover is called overcooling. The cold surface water quickly mixes with the warmer water in the deeper parts of the pond where your fish have been overwintering, causing the overall temperature of the pond to drop. 

This sudden temperature change can shock fish and lead to fish kills. Typically, an aerator is the only water feature you want to leave running after you shut down your pond. 

Insufficient oxygen levels 

While fish live in water, they also need oxygen to live. Fish absorb dissolved oxygen through water using their gills. If your pond doesn't have enough oxygen, it can cause water quality issues and eventually lead to dead fish. 

Ponds release gases and absorb oxygen when the pond is circulating, primarily through a water feature. As a fountain sprays water in the air, water comes in contact with oxygen, and gases like C02 are released. Running your pond's filtration system and water features 24/7 helps to keep your pond oxygenated.

In shallow ponds, the wind will naturally aerate the water's surface. Shallow ponds tend to warm up quickly in the summer without shade or pond tint. Warm water can hold less dissolved oxygen than colder water, making it more difficult to maintain oxygen levels. 

The right aquatic plants help to filter out nutrients in the water and release oxygen in the process. You can use a combination of floating, emergent, and submerged plants to decorate your pond while also helping to reduce nutrient loads. 

Algae blooms

While some aquatic plants help oxygenate the water, an algae bloom can have the opposite effect. Algae blooms can quickly take oxygen from the water, causing the levels to drop. Some amounts of green algae in a pond are normal. They bloom when they have access to nutrients like phosphates and nitrogen and start causing water quality issues.

If the blooms take over the water, the algae can even start clogging your fish's gills. Some algae, like blue-green or red algae, release toxins into the water that can cause fish kills. 

Poor water quality  

Regularly testing your water is helpful for making sure that everything is balanced and there are no contaminants in the water. If your pond has too high levels of ammonia or your pH is off, it can lead to fish kills. Sometimes, water quality issues aren't obvious unless you're closely observing your fish and noticing any odd behaviors. 

When adding fish to your pond, the water may look clear and healthy, but tap water has high levels of chlorine. Before adding any fish, you'll need to use a chlorine remover first. 

Once you start seeing signs of water quality issues like cloudy water or unpleasant smells, you'll want to use a water treatment and determine the cause so you can prevent it from harming your fish. 

Fish sickness

Just like any other pet, fish can get sick too. Being on the lookout for unusual behaviors can help you identify your sick fish before it spreads to all of them. If you have just one or two sick fish, you can remove them from the pond and treat them in a temporary tank. Once three or more fish show symptoms, you'll want to treat your entire pond. 

Common illnesses in pond fish: 

  • Fin rot
  • Parasites (ex., white spot, carp louse, skin worm)  
  • Dropsy
  • Pop eye

Understanding the common causes of fish kills can help you prevent them from happening in your pond. Routine pond maintenance is a great place to start. Testing your water and making sure you have proper aeration and filtration can prevent many issues that cause fish to die suddenly. Keeping an eye on your fish and looking out for any unusual behaviors and signs of sickness can help you catch parasites and infections and treat them in time. 

Meta: Discover the most common reasons for fish kills and how to prevent dead pond fish. 



Your cart is currently empty.

Start Shopping

Select options