Why your pond fish keep dying – With Solutions

Last Updated on July 4, 2021 by cmoarz

It’s not uncommon for people to get frustrated when they try to grow fish in their pond and find they keep dying. They may have tried a few different things, but nothing seems to work. The problem is usually much simpler than they think – let’s get into the most common way your pond fish are dying suddenly.

Tl;dr for skimmer

Bottom line, Your pond fish are likely dying from low dissolved oxygen levels from overcrowding or an algae bloom, Ammonia, and nitrite levels may have spiked, or there has been a large temperature fluctuation killing your fish.

Overcrowding and overfeeding

The most common reason you may be losing fish is this: You put too many in there, to begin with. They grow, they breed, and now you have a lot of small fish that are eating a LOT more than you think, and their bioloads are starting to add up.

Even if they didn’t breed just yet, the bigger they get the larger that bioload is going to be, and the more resources they are going to take out of the water in order to sustain themselves.

This poses several issues:

Dissolved oxygen levels go down

Oxygen is what fish need to live, like just about everything else alive. So if they don’t have enough of it in the water, they are going to die.

If your pond has a lot of fish in there it could be causing an uptick in the amount of oxygen being used. This compounds as the fish get larger, and breed.

You might not see the dead bodies floating up right away but watch for other signs like jumping out of the water (they’re trying to get air), or staying at the surface all the time gasping.

Low oxygenation caused by an algae bloom is also something to be aware of.

An easy fix to this is to add additional aeration in the pond and consider making sure you haven’t overstocked it.

When Ammonia / Nitrite Spike Up , Fish Go Belly Up

When a goldfish/koi eats its food it’s digested into something called ammonia. This is toxic to any aquatic life. Normally in a healthy pond, bacteria will naturally break down any ammonia into nitrites and nitrites into nitrates, which don’t poison outside of large amounts.

However, if there are too many fish, or if the bacteria aren’t able to break it down fast enough because it is just too much, then you’ll start to see a buildup of ammonia.

This isn’t something that can be fixed instantly, and removing some of the fish (or adding more aeration) may help with this as you are reducing the amount of “grazers” so less food is being broken down into toxic ammonia.

It usually takes about 5-7 days to resolve on its own, though it’s a good idea to make sure your bio filter (if you are using one) is strong enough to handle all those extra fish.

Not all ponds have filters, But you should consider getting one if excess waste is an issue.

If you are concerned about the sudden death of fish in your pond or have other concerns about water quality, then take a sample to your local pond store to see if they can help. They will be able to tell you what might be affecting your fish and how to address it.

Home test kits are also an affordable option, check out some of these selections we recommend:

Water Temperature Fluctuations can cause mass dieoffs

Koi are coldwater fish, which means they like cool temperatures best. It’s not uncommon for people with outdoor koi ponds to notice large amounts of dead koi during one season when their weather gets warmer than they’re used to (e.g., an extreme summer).

This is usually due to them getting too hot because the pond isn’t being properly maintained. Does the pump run all night? Is there enough surface area in the pond for the whole thing to cool off when it gets too hot outside?

Have you considered installing a chiller? (Depending on how big your pond is this may not be an option for 99.9% of you)

If they’re all killed, then this is probably the problem. If it’s just a few of them, then maybe there’s something else going that we haven’t assessed yet.

Consider several temperature probes attached to an alarm on your phone or in your home that goes off when they reach certain temperatures. This can give you enough time to cool the pond down if necessary with large chunks of ice, or raising a shade cover.

Just be careful not to lower the water temperature too quickly and cause shock to the fish. This is fairly hard to do in a large enough pond, unlike an aquarium.

Disease can kill goldfish and koi one by one

Although your fish may seem to be dying suddenly, it’s possible they’ve actually been sick for a while. Sometimes you’ll notice that some of their scales are slimy or look a little funny. These are signs of disease.

They release waste products into the surrounding tissues and bloodstream which can affect other organs if not treated promptly with medications.

It’s also possible that one of your fish has an illness that is contagious to others, and in those cases, you need to be able to act quickly before the entire population is wiped out by something like furunculosis (often called “skinny disease”).

This could be a cause if your goldfish are dying one by one, same with koi. If there are no obvious symptoms but a few fish just keep dying randomly over several days, consider taking samples to your local pond store testing.

Pesticides or Medications in the Pond Is a Common Killer

Certain medicines that you might use on your property can harm fish. For example, if you were attempting to treat mosquitos with pesticide and some of it got onto pond water, your fish would die.

If a neighbor uses fertilizer in their yard, again you run into the same problem (as well as dirty water to begin with). All it takes is one exposure for them to die. So be careful what kind of substances come close!

You can build a barrier from runoff and attempt to keep the pond completely isolated from ground water.

There is only so much you can do, however.

Koi ponds are a precious home asset, and it’s important to take care of them. We hope that you have found this article on preventing and diagnosing goldfish and koi pond deaths helpful.

If you have any questions or comments, feel free to leave them below.

Here are some related questions that might prove useful to you folks as well.

Do koi fish float when they die?

Sometimes they can float and other times they can sink. It comes down to how they died and if they had a full swim bladder.

Could the dead fish in my pond have been killed by parasites?

Parasites are always a possibility. However, if your koi are dying and you’ve thoroughly checked for them and don’t see any obvious signs of infestation, then it’s probably not this one. Always better safe than sorry though and you should talk to your local aquatic vet.

More commonly than not, they would be exhibiting other common symptoms like loss of appetite or thickening of scales that most people notice before there are any deaths involved.

Why is/are my koi staying in one spot?

There are several reasons for this, most of which have already been stated in the article above.

  • Low oxygenation from overcrowding causing the koi to stay at the top of the water to breathe better.
  • Low oxygenation caused by an algae bloom.
  • Toxicity from certain materials in the pond causing the fish to stay near the surface of the water.
  • They’re just waiting for you to feed them.

Koi have also been known to come to the top of the water if you are standing there because they’ve been trained to do this when you feed them every day.


Owner of AquariumGravel.com and also owner of actual Aquarium Gravel believe it or not! ;). Setting up beautiful aquarium sceneries and habitats since I was very young. Enjoy!