Why Are My Pond Fish Dying After Rain? (How to Prevent it)

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Ponds are sensitive ecosystems and as such, fish die offs can occur for a variety of reasons. Even something as simple as the pond water warming up too quickly or the pH being altered by just one or two points can cause issues (most common pond fish species prefer pH to be between 7 and 8).

Hot days, cold snaps, a reduction in the amount of dissolved oxygen, or even excessive rainfall can lead to some major problems with your pond, as pond fish such as goldfish and koi require specific habitat parameters to survive. When water quality is altered even by a small degree and especially if this happens quickly, fish can become lethargic, ill, or even die.

Why is Heavy Rain Dangerous to Pond Fish?

Many pond owners have woken up the morning after a rainstorm to find that all or many of their fish have, seemingly mystically, died. This phenomenon occurs in natural water areas as well, and for the same reasons as in your pond.

1) Rainfall Alters Water Chemistry


rainfall alters water chemistry in koi goldfish garden ponds
Public domain.

Rainfall is typically rich in nutrients, due simply to the nature of the water cycle. During the cycle, most water will ordinarily pass through soil, plants, the bodies of animals that ingest it, and the atmosphere. As such, rainwater typically contains sodium, phosphorous, calcium, magnesium, nitrogen (primarily from passing through the atmosphere), and potassium, among others, as well as any pollutants that may be present in the soil or atmosphere.

Because of these nutrients, gardeners generally love rainfall for their plants, but when it comes to ponds and other water bodies, too much rain can deposit too many nutrients too quickly, thus overloading the system before it can filter through the nutrients, altering pH and water hardness, and leading to sudden fish kills. Sudden changes in pH can, for instance, deplete the protective mucous layer covering fish gills, making fish vulnerable to illness, lesions, and parasites.

2) Heavy Rainfall Causes Turnover

heavy rainfall causes turnover in ponds
Public domain.

During summer and early autumn, warmer, more oxygen rich water is found toward the water’s surface and cooler, anoxic waters with less oxygen are found at the bottom of deeper water bodies such as lakes and larger ponds. Typically, rain is cool or cold, and cold water is denser than warm water. When a storm dumps a great deal of cold, dense water into your pond, it settles to the bottom, displacing the low-oxygen water at the bottom of the pond and causing it to turnover to the top of the pond where fish are typically found.

Not only does the temperature of your pond suddenly shift, which in itself is quite dangerous for fish as it can cause paralysis or even death, but now your fish are surrounded by water with very little oxygen. After rainstorms, many fish simply suffocate to death. Strong winds, even without rainfall, can lead to this hazardous turnover as well. Sudden turnover also often causes sediment to stir up, leading to cloudy water that’s hard on your fish.

3) Rainfall Can Physically Wash Fish Away

fish can wash away out of ponds during heavy rainfall
Public domain.

If too much rain falls too quickly, flooding will of course occur as the soil will not be able to soak up all of the moisture. As such, sometimes heavy rainfall can flood your pond and actually wash your fish out of it. If this happens while you’re around, then great – there is potential for you to catch your fish and place them indoors in containers or tanks. However, if you’re away during a storm and this happens, it’s not uncommon to find fish beached on land after a hefty rain, which of course kills them if they are unable to somehow find a way back into the water within a few hours.

4) Substantial Rainstorms Cause Runoff

harmful runoff into pond from heavy rain
Public domain.

This one rather ties flooding and water chemistry together. During an extensive rainstorm, water will flow down gradient as the soil will not be able to soak it all up quickly enough. The water picks up all manner of nutrients and pollutants as it flows across the land, and these nutrients and pollutants will then be deposited wherever the water stops flowing – such as into your pond.

This is of particular concern if you live near an agricultural area (the primary concern being waste from cattle as well as fertilizers), some sort of industrial site, or in/near a city where water will be able to pick up an assortment of pollutants (such as leaked motor oil from cars). All of these excess nutrients and chemicals will alter pH, deplete oxygen, and feed potentially harmful algae, leading to suffocation and fish kills in extreme cases.

The Best Ways to Protect Against Heavy Rains (Top Methods)

1) Provide Constant Aeration & Oxygen

An aerator keeps ponds clean after heavy rainfallThough incorporating aerators into your pond won’t eliminate all dangers from heavy rainfall occurrences, it will help to make sure that oxygen levels stay at a healthy level in the event that turnover occurs.

If you don’t already have them, install at least one aerator as well as plenty of plants to help oxygenate the water. Water movement will also aid in providing dissolved oxygen (so long as the water movement isn’t too fast or choppy, as pond fish prefer still waters), so having a small waterfall, air pump, or fountain can help keep water mixed up and aerated, thus minimizing the impacts of any turnover occurrence that may transpire.

2) Set-up Strong Fencing & Barriers

Fencing and barriers prevent fish from washing away during heavy rainThis one may sound a bit strange, but installing fencing is a relatively simple and straightforward way to help protect your fish. In the event that flooding occurs, the fence will keep them inside of the pond and off of your lawn.

Alternatively, you could raise the rim of the pond several inches above the ground – this would both help prevent your fish from being washed out of your pond, but would also help prevent debris such as dirt, grass clippings, and trash from washing in.

3) Install Drainage or Overflows

Runoff skimmer system protect fish from heavy rainfallTo maintain stable water levels during heavy rain, you can install some form of drainage or runoff system. To achieve this, you can install an actual drain (such as a grate with rocks) at a low point near your pond or at a high point on a wall within the pond. Another option is to aerate your soil by poking small holes in it (such as farmers often do in their fields) so it has an easier time soaking up water, or creating your own little wetland/bog area just outside of your pond. Wetlands are nature’s purifying sponges, soaking up water and nutrients and effectively filtering them. Alternatively, using a pond skimmer slightly above the water line connected to a drainage pipe should work well to both reduce excess water and also manage debris.

You can also incorporate a slope on at least one side of your pond, so that excess water will flow out and prevent flooding. Whichever side you have the slope on, you may want to install fencing to prevent your fish from potentially washing out. On a similar note, if you’re creating a pond, make sure to place it at the high point on your property to minimize the risk of flooding.

4) Reduce Pond Total Bioload

Pond filtration system reduces bioload too many nutrients after rainReducing the bioload on your pond is fairly simple. Limiting fertilizer use near your pond (as well as any chemicals in or around your pond), not clipping grass within a meter or so of your pond’s edge, and regularly cleaning leaves and sticks from your yard will cut down on the amount of stuff that will wash into your pond during heavy rainfall. As well as this, you should be have installed a filter system sufficient for your fish stocks which is regularly cleaned and maintained to a high standard. In turn, all these methods provide a lower risk of overloading your pond and altering the water quality beyond what your fish can handle.

12 thoughts on “Why Are My Pond Fish Dying After Rain? (How to Prevent it)”

  1. Very informative, thank you!
    I have anew “pond” that i built which is roughly 3 months old and is getting established with some fish, plants, bugs, ect. It’s only about 150 gallons and it’s been fighting tannin stained water. Well we just had 2-3” rain and the pond went super dark stained with tannins. I cleaned the filter and turned on the fountain to help circulate more water.

  2. I built a pond back in February and had the same problem eventually.
    In the beginning after things had settled for a couple of weeks or so, I introduced a lilly planter and some reeds in a planter, then put in a couple of sticklebacks and Stoney loach. The water then seemed to go through a transformation, first the tannings, then the greenish water. I could just say that the waters natural cycle was at work, but I did things to help it along. I changed my solar powered pump from my waterfall to just a fountain (for occasional use) and purchased an upgraded eco pump for the the waterfall (that runs constantly)
    Having a spillbox blade type waterfall I eventually found the right balance of filter with some matting added to catch the finer particles, I haven’t cleaned or removed the meshing for well over a month now and my little pond is gin clear, previous pond clear treatments only hindered the cycling process or maybe enhanced it to be fair with perseverance of sticking with the adequate filtering biology

    • Thanx for sharing this I have only had (2 koi and a coment ) fish in my pond for a week However this weekend we’ve had over a 100mm of ran and only the one Koi is still energetic the other 2 are very lazy looking and I’m wondering if it has something to do with the rain

  3. One hundred and more goldfish of mine died after feeding my goldfish on straight rainy days. The dead goldfish floats and with bloated stomachs. Although I provide them with good aerator, ten to twenty per cent of water change and quality food. But still, in the morning, I used to collect dead goldfish and it broke my heart. Thank you for the knowledge you have shared. God bless all of us, fish keepers.

  4. Very informative. I had 20 goldfish of five years In pool. All 20 died overnight last Friday 21-22 October after big rain storm. Located in Staffordshire.

  5. I put water in my pond to bring the level up. I am in NZ and it’s summer here. today I found all my goldfish belly up. My son thinks the addtion of water would have changed the chemical level of water in the pond. Is this true?

    • Hi Gay,

      We’re so sorry to hear that! Adding water does change the water chemistry of the pond; this is particularly true if your water has chlorine in it. Any new water added should be dechlorinated (there are simple garden hose attachments with carbon that can dechlorinate water directly as it runs through the hose), and also allowed to sit if possible to rise to the temperature of the rest of the pond. Both of these methods greatly reduce potential shock to fish, as goldfish and koi alike are sensitive to chlorine and sudden temperature changes. I would also recommend testing your water quality parameters both before and after adding water to the pond, so that you’ll know if things like water hardness, pH, and so on are off. Then you can take measures to help avoid harming fish.

      Hope this helps!

  6. Read your article. Comes to me right after 2 heavy rainstorms with ensuing floods of the pond.
    Everything was clearing until the floods, then dirt and crap everywhere. Plants washed out, dirt all over but the fish survived (thankfully) and I had two waterfalls plus a pressure filter to clean things up. I added synthetic batting to the filters in the waterfalls as well as a makeshift batting filter to clean up the dirt. Seems to have worked. Water is clearing, the fish are happy. Unfortunately, so are the frogs.
    Thanks for the info!

  7. Hi, we have a large natural pond (9mx9m) and the levels usually go up and down with water table levels. In winter the pond is about 6ft deep and with the drought it has dropped to 3ft. We have a carpet of oxygenaters on the bottom. We had a couple of brief but heavy showers over the last couple of days. Unfortunately, even though it is a natural pond it has goldfish in it and yesterday we found 4 dead fish and today we have fished out another 15. Would this be the rain and should we be testing the pH levels as well?
    Many thanks


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