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Why Are My Pond Fish Dying? (5 Potential Causes)

Why Are My Pond Fish Dying? (5 Potential Causes)

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Sometimes pond fish die out of a natural process. If you occasionally see a few dead ones, it’s not a cause for alarm, as it could only be due to old age, predation, spawning stress, and the like.

However, if the fish continue to die suddenly, you’re likely looking at some pond issues you need to deal with as soon as possible. Having them die in large numbers can severely affect the pond’s ecosystem and the remaining living ones.

Before you scratch your head in frustration, asking, “why are my pond fish dying?” we’re here to help you get to the root cause. Here are the possible reasons why your pond fish are dying and what to do with them.

What Causes Pond Fish to Die Suddenly?

There are many reasons why pond fish suddenly die and float on the surface. While some are due to natural causes, which are to be expected at times, other reasons involve more pressing concerns.

In such cases, it’s best you observe if you’ve made any major adjustments to the pond that may have affected the fish. These are the common causes of pond fish death you should be on the lookout for:

1 – Summer Pond Fish Kills

The sudden death spikes in pond fish during summer often relate to dry weather conditions, changes in water temperature, and algal blooms. The warmer the pond’s water gets beyond fish tolerance, the higher their mortality rate increases.

You’ll notice this occurrence when the pond fish start to gulp at the water’s surface and creatures like snails and crayfish leave and gather ashore. Once it starts, it can be borderline impossible to quickly remedy the situation and prevent further fish kills.

The best way to avoid summer fish kills is to take precautionary steps on or before the summer season starts. These steps include the following:

  • Reducing algal growth by using herbicides (with moderation), planting aquatic plants, and adding barley straw
  • Installing aeration devices such as fountains and water circulators to maintain oxygen levels
  • Avoiding overcrowding in the fish pond
  • Sustain the pond’s water by regularly conducting water quality tests

2 – Winter Pond Fish Kills

If you’re living in areas with colder climates, your pond may be susceptible to winter fish kills. This happens when ponds freeze or get covered by excess snow for long periods.

Shallow ponds with too much plant life are common victims of winter fish kills. The more aquatic plants you have in your pond during winter, the more oxygen is used rather than produced.

Similar to summer fish kills, the key solutions are done in preparation for the winter season. Here’s how you can lessen the risks of winter fish kill:

  • Reduce and control plant growth before the winter season to avoid oxygen depletion
  • Install aeration devices to balance oxygen levels
  • Dig some trenches in the pond to add depth and water volume
  • Regularly plow the snow in your area

3 – Pesticide Poisoning

Unlike summer and winter fish kills, pesticide poisoning can be a bit harder to track and determine. However, there are visible behavioral changes within 24 hours of adding pesticides to your pond.

Whenever you apply pesticides to your pond, you should always be on the lookout for the following symptoms:

  • Fish coughing and convulsions
  • Sluggish and uncoordinated movements like swimming in circles
  • Involuntary muscle contractions
  • Considerable amount of fish death, especially with smaller fish

In cases of pesticide poisoning, it’s best to immediately seek help from a professional toxicologist or industrial hygienist. Your health may also be at risk if you try to fix it by yourself with little knowledge.

4 – Overfeeding and Overcrowding

Overfeeding and overcrowding are always a cause of concern for fish ponds. Since there’s not much room to work with for the fish, there will be lesser oxygen available and increased waste amount.

The clearest sign of an overcrowded pond is if its nitrite and ammonia levels reach beyond 0 ppm (parts per million). You can check this by running water quality tests.

You can prevent overcrowding by ensuring that the pond’s water meets the 0 ppm standard. As for overfeeding, the general rule is to remove any leftover fish food after five minutes.

5 – Pond Fish Disease

The most common cause of diseases in fish ponds is a compromised immune system due to stress. While it’s also possible for fish to die because of stress, it often leads to developing diseases.

It’s usually environmental factors like overcrowding and poor water quality that cause stress to fish. As a result, they become susceptible to various diseases including the following:

  • Dropsy
  • Fin rot
  • Mouth rot
  • Ulcer
  • Pop eye disease
  • Tail rot
  • Gill disease

In most cases, you’ll have no choice but to let the disease run its course until it dies out. If you want to lessen the risk of such complications, ensure that your pond’s water quality is sustainable for the fish and avoid overcrowding.

Can Lightning Kill Fish in a Pond?

While it’s definitely possible for pond fish to die due to lightning strikes, it doesn’t happen too often. As much as water is a good conductor of electricity, the electric current travels across the surface rather than deeper underwater.

If your pond is unlucky enough to get directly hit by lightning and there are fish in the affected area, then yes, they could die. However, since most fish dwell deep in the water, the most they can feel are tingles.

Will Chlorine Kill Fish in a Pond?

Chlorine doesn’t necessarily kill fish instantly, but it comes with short to long-term harmful effects. However, concentrated levels of chlorine can definitely prove to be lethal.

If you wish to chlorinate your pond, you shouldn’t be fish and aquatic plants in it. Here are some of the harmful effects chlorine can do on your pond fish:

  • Causes stress and diseases
  • Damages a fish’s gill structure, which leads to breathing problems
  • Chlorine becomes more toxic the lower the water’s pH level becomes
  • Increases the mortality rate of fish and other aquatic plants

Can Algae Kill Fish in a Pond?

The presence of algae in fish ponds is generally harmless and beneficial to their ecosystem. However, a sudden surge in algae or algal bloom spells bad news for your pond and its fish.

When there are too many algae in your pond, nutrients such as nitrogen and phosphorus tend to spike. Since there’s an increase in these nutrients, algae reproduction is encouraged.

Additionally, your pond may be prone to increase bacterial decomposition and oxygen depletion the more algae it has.

Do Cats Kill Pond Fish?

While it’s certainly possible for cats to prey on pond fish, it doesn’t always happen. It all depends on your cat’s behavior and how you raise it to interact with other animals.

Since cats are also curious creatures, they might be tempted to drink from your pond. The best way to prevent cats from coming near your pond is by adding physical deterrents like barriers, decorative boulders, and fences.

You’d also want to socially train your cats so that they’ll avoid messing around with your fish.

Will Pond Dye Kill Fish?

People often use pond dyes for decorative purposes and sometimes to reduce algae growth by decreasing the pond’s photic zone as it receives sunlight. While this may seem like good news, it can be problematic if too many algae die at once, causing a sudden spike in bacterial decomposition.

Overall, pond dyes aren’t harmful to fish, so long as you stick to quality and trusted brands.

What to Do With Dead Fish in a Pond?

If you spot a dead fish or two in your pond, you should dispose of them immediately. Once their body decays, they’ll emit bacterial wastes that can affect other living fish.

While it’s common for people to flush dead fish down the toilet, you shouldn’t follow suit. Doing so can sometimes cause drainage problems, especially if done often or in large amounts.

The best way to dispose of dead fish is by burying them in your garden. You can also add them to a compost bin for fertilizing purposes.

Final Thoughts

It’s always terrible for fish breeders and owners to have their pond fish suddenly dying. Luckily, there are many ways you can prevent the avoidable situations mentioned above.

Remember that it’s easier to prevent fish kills than deal with it as it happens. Keep the waters clean and healthy, and you’ll have yourself a pond that’s brimming with life.


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