Why Are My Fish Dying In My Outside Pond?

Dead Koi Carp in Outside Fish Pond

You may find that your fish are lying there floating upside down in your outside pond leaving you completely stumped.

The number 1 reason why fish die out is poor water quality. It sounds obvious, but when it comes to outdoor fish ponds a lot of people have a tendency to ignore their water quality, whereas, in an aquarium, you’ll be monitoring your water quality fairly frequently doing good water changes.

As long as the water stays clear, people seem to think that the water quality is quite good, but this certainly isn’t the truth.

Below are the most common reasons why your koi carp or goldfish are dying out in your garden pond.

Lack of Oxygen

This is one of the major reasons why your fish will die. This is more in the summertime when temperatures are hot, but it can actually strike at any point, and that is a lack of oxygen.

Fish obviously need oxygen to breathe. When there isn’t enough oxygen in your pond, your fish will simply die because they will suffocate. In warmer temperatures, there is less oxygen dissolved in your water and your fish can actually suffocate.

During the summer months what you need to do is have some air stones, a fountain, or some kind of waterfall to increase the oxygen levels in your pond. Keep your water turning over, otherwise what will happen is that the fish will actually start to come to the surface, gasping for breath. If this happens and things aren’t resolved, the fish will simply die out.

A lack of oxygen can also be caused by an algae bloom. This can happen when algae suddenly grows due to increased nutrients with the pond turning green, or you get a massive amount of unwanted blanket weed. This will suck out all of the oxygen from the water and your fish will die from lack of oxygen this way.

Bad Filtration

Filtration has a big part to play in keeping your fish healthy. It is the heart of your pond. It will break down all the waste that your fish produce and any food that isn’t eaten or is rotting in your pond or any leafs that fall into it. A good filter is what is going to break stuff down and keep the water quality good.

Everything your fish eats comes out the other end, and it comes out in an organic form. This form is toxic, it’s called ammonia and it binds to fish gills and stops your fish from being able to breathe.

In a healthy pond with a good mature filter, the ammonia is quickly broken down into nitrites and then into nitrates. This doesn’t actually affect your fish in a negative way because it is now in a safe, stable form of nitrate. This process doesn’t really happen, or it doesn’t happen quickly enough and your fish will end up at the surface of the water on their sides, gasping for breath. By this point, the ammonia levels have likely gotten too high.

High ammonia levels are one of the biggest killers of fish after a lack of oxygen. This is very common in the colder months, such as autumn or winter, where your pond bacteria actually isn’t working very well. You’ll also find that if you keep feeding your fish, the fish food will break down into ammonia, producing a spike in levels, causing fish to die.

This is one of the most common reasons why fish die over the winter, simply due to the fact that people don’t stop feeding them. So if you spot your fish on their sides, or they’re struggling at the surface for air, and you have ruled out that it’s not just simply a lack of oxygen, then do an ammonia test on your pond. If you find ammonia then you need to add an ammonia remover and increase your filter and the filter bacteria.

Use of Pesticides or Herbicides

Another reason why your fish could be gasping at the surface or laying on their sides looking very ill, possibly already dead, is something like pesticides. Pesticides and herbicides are commonly used in the garden, and many of them are absolutely toxic to aquatic life. If you’ve just recently killed some moss or weeds, or have sprayed your plants with pesticides, this can easily make its way into your pond system and will kill your aquatic life extremely quickly.

If you suspect any pesticides or herbicides have made it into your water system, you need to remove your fish from your pond immediately, then add some declorinator or something similar to help break some of it down. Put the fish into some fresh water with good levels of oxygen and hopefully, they will be swimming around as normal.


Diseases don’t always come on their own. They’re normally triggered even when a healthy fish has a strong immune system and tends to not be affected by things like flukes, bacteria, and other things like that. It’s normally when their immune system is lower due to a stressor, that they tend to get these diseases, but this is not always the case. Things like flukes and parasites can actually be brought in from when you add new fish to your pond, or, they can sometimes be brought in by other things like birds from ponds that have got an infection or disease. So new entries to your water system need to be considered here.

Flukes and parasites aren’t always apparent on your fish. They will often be almost undetectable and sometimes you have to do something called a scale scrape. This is where you scrape your pond fish and look at it under a microscope to see what kind of flukes or parasites are present on the fish.

Flukes and parasites are bad for your fish because they simply weaken them. They tend to be an irritant on the fish’s scales, but they will also remove nutrients from the fish themselves, weakening them and stopping them from using the energy in their body. What you will notice is that they’ll just generally get weaker and weaker over time. Normally once a parasite is on a fish, their immune system will go down, and then they’ll start getting other things such as ulcers. They’ll also start getting secondary bacterial infections, where things will really go downhill from then on. Keep doing constant visual checks on your fish for these potential problems.

If they have any signs of milkiness on their scales or they’ve got patches of discolouration, are breathing heavily, or are crashing off things in the pond, then you need to have a good look at them and see if they’ve got any parasites or flukes on their scales. There are lots of treatments out there to get rid of the flukes and parasites. Treating it is very simple, but you need to make sure you do it before it gets out of hand, because it can cause very fast and sudden wipeouts in ponds. Normally things like ulcers are actually secondary infections from flukes. So the fluke will sit on scales and it’ll do its thing, then the wound that the fluke’s causing will get infected by a bacteria. The bacteria will eventually start to eat away at the fish and cause ulcers.


Predation is very common in UK ponds, with the main culprit being your friendly neighbourhood heron. There are other things that can eat your fish, such as cats, dogs, otters, and foxes trying to get an easy meal.

Most of these animals are actually protected by law in the UK, so you can’t go out and shoot your heron or your otters to try and protect your fish. You have to just protect your fish directly by using deterrents such as netting, tiles, decoys, etc.

You can get tiles that will go around the edge and will stop the heron from entering your pond. One of the best tactics you can use is to install an electric fence around your pond, this will stop most animals from going near the edge of your water. It will also discourage herons from landing on the edge or any platforms at the side of your pond for a quick look in.

The best course of action to combat predation is a combination of different deterrents, all depending on your setup and level of threat.

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