Oh No, My Pond Fish Did Not make It Through the Winter!

Oh No, My Pond Fish Did Not make It Through the Winter!

Posted on Apr 7th 2020
Author: Bill Hoffman

The weather is beginning to get warmer and I started to open my pond for spring, but there’s a problem... My koi and pond fish did not survive the winter! What happened?!

Right away I’d like to express how sorry I am. After all, your finned friends were your pets and part of the family. Hopefully I can shed some light on things in order to help prevent and future fish loss while maybe bringing some understanding and closure.

Let’s start troubleshooting and see what happened to your pond and fish over the winter months.


Did a hole stay open on the surface all winter?

  • Check to make sure your pond de-icer (pond heater) didn’t fail.
    • Melting of its plastic case or having water inside the unit may be a good indication that it did.
      *Most de-icers only have a one-year warranty, so do not wait until next year to get it replaced.
  • Check to make sure your GFCI is not tripped.
    • What can cause this to happen you might ask? Moisture in an outlet or an extension cord, a failed pond de-icer, or sometimes rodents will chew through wiring underground - making it difficult to find the problem.
    • If you are not able to get it to reset, first check your breaker box. Second, check your wire leading to the outlet.
  • As we all know, the weather in Ohio can be very unpredictable. Extreme temperatures and heavy snowfall are unavoidable.
    • Make sure that your pond heater stays afloat on the surface and does not get under the snow or pushed under the ice.

Keeping a hole open in your pond's surface is very important to the health of your fish in winter.


How much debris is on the bottom of your pond?

  • Having a lot of leaves in your pond can rob pond water of oxygen during decomposition, and also invite a fungus that is contagious to your pond fish.
  • Pond plants that have not been cut back or removed from the pond can add to the oxygen depletion and muck build up.
  • A thick buildup of muck in the pond can harbor bad bacteria and parasites that are harmful to your fish.
  • Fish are cold blooded. This means their body temperature is the same as the water they are in. This will lower the immune system and make them more susceptible to health problems:
    • If you see white “fuzz” on their skin, that is typically a fungal attack. However, fungus is usually a secondary infection after they have already been compromised.
    • External bacterial infections show up as pink streaks and/or ulcers on the skin, cloudy or bulging eyes and/or the tail or fins “rotting” away.
    • Parasites are more difficult to identify, especially without a microscope, but there are a few that can be seen with the eye; ie. Fish lice (a roundish clear spot on the surface of the skin with tiny black dots in it), Anchor worm (look like a tiny white thread coming from under a scale) and Ich (appears as tiny white spots, as small as salt or sand, covering the body of the fish.

How much debris is in the bottom of your pond?


Did you have too many fish?

A heavy fish load can be very dangerous for many reasons:

  • Buildup of ammonia from fish waste settles to the bottom of ponds, which is where your fish go seeking warmer water.
    • When the pond surface freezes, oxygen is limited to getting in by only the hole open in the ice. A pond aeration system can help greatly in this situation by putting oxygen in under the ice. Install at approximately half the depth of the pond so as not to lower water temperatures at the bottom.
  • Having more fish in your pond water puts a higher demand on oxygen levels.
    • Ammonia can burn their gills (organ with which they get oxygen), the way smoke from a house fire can burn our lungs. The more fish, the more fish waste. Ammonia is broken down by beneficial bacteria, but the cold water slows this process way down.

Don't over crowd your pond. It could lead to problems in your pond.

*How many fish is too many in my pond? We have a blog for that here.


Did you feed them too late or during a warm spell?

  • Colder water [below 50 degrees] means fish are not able to digest food like they do in warmer water.
    • With fish, the food literally goes in one end and out the other. They have no stomach, just a digestive tract that absorbs the nutrients as it gets broken down along the way.
    • As temperatures drop, their metabolism slows down. This means that the digestion of its food slows and could possibly stop in their digestive tract.
    • Processed fish food, higher in protein, that sits all winter in their digestive tract, may cause an ulcer and bad bacteria inside the pond fish.
    • As the water warms up in Spring, things start to move through its system again, but this time, bad bacteria moves into its organs instead of out the other end.
    • In this instance, you will usually see that the fish start dying off in the spring.

*For tips on feeding your koi and pond fish take a look at our “When to Stop Feeding your Fish for Winter“ blog.


How did they all just disappear?!

  • Unfortunately, cold weather means we do not have the benefit of coverage from water lilies and marginal pond plants to help protect our finned friends from the many predators who stalk our ponds.
    • Blue heron and raccoons are menacing, but usually hibernate through most of the winter. Plus, your trash can is easier hunting for a raccoon rather than the frozen pond.
    • Owls and hawks can swoop in and fly off with a fish, unless the pond is covered with ice.
    • A mink or river otter, however, can sneak in through the opening in your ice, swim around the pond and are known to wipe out the entire population before you are any the wiser.

Owls, Mink, raccoons and blue heron are all pond predators that can sneak fish from your pond.

If you need any help or pond supplies or accessories please feel free to contact us or visit us at our Uniontown, Ohio location. You can always buy pond supplies online too. For pond fish food and care items click Here.