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What Could be the Reason for all the Fish Dying?

By: Dr Gareth Evans - Updated: 27 Mar 2021 | comments*Discuss
What Could Be The Reason For All The Fish Dying?

Q.My mother has maintained a pond (with great success and affection!) for many, many years. However, in the last 4 days 11 of her fish have died.

A friend has mentioned that berries from an adjacent coneaster bush may be the problem, although the bush has been there for years also. Do you know what the problem could be?
(Ms Claire Gunner, 25 September 2008)


As they say in CSI, we’re a bit short on forensics; we have the unfortunate victims and a suspect – in this case a Cotoneaster bush – but not much in the way of other evidence!

Cotoneaster berries – and specifically the seeds – are known to be a mild stomach irritant to humans and there are American studies which suggest that some wild birds eating large numbers of them on a long term basis can be poisoned – so your friend may have a point. However, I’ve a niggling suspicion that something else is going on.

Lack of Oxygen?

Assuming that there’s no obvious signs of damage or disease on your mother’s fish – the most common cause of mass fish deaths is lack of oxygen. Now I don’t know how many fish there are in the pond – or what kind they are – but if the unfortunate 11 represents all or nearly all of them, poor oxygenation could be one likely cause. If your water recently went green, or a large amount of plant material has found its way into the pond – and this might include those Cotoneaster berries – the breakdown of the algae or other vegetable matter could have taken the oxygen out of the water. The same might have happened if the water level became unusually low or your pump packed up – but without knowing more of what the cop shows call the “circumstantial evidence”, this is only speculation at best.

Water Quality

It is probably worth testing the water for all the usual things – ammonia, nitrite, nitrate and pH – if you haven’t already. This might give some clue as to what has happened and it may also be a good idea to clean out any settlement residue in the filter and change around a third of the pond’s water to be on the safe side.

If this still leaves you none-the-wiser, the only thing left to try is to cast your mind back over the last few weeks and see if anything out of the ordinary has happened. Have any pesticides or fertiliser been used around the pond, have you changed your brand of fish food or dosed your pond with some treatment or other? It is a bit of an exercise of grabbing at straws, but it might just make things a little clearer.

Fish Factors

The fish themselves might also be a factor in this; if they, like the pond and our prime suspect, the Cotoneaster bush, have also been around for a long while, their advancing years may have played a part.

Now I’m not suggesting that they all died of old age within a couple of days of each other – but like humans, as fish get older, their natural resistance diminishes and their ability to bounce-back drops. Their age could have made them less able to cope to some change in the pond that you might not even have been aware of – but without knowing a bit more about the fish themselves, this is pure speculation again!

The bottom line is, you may never know – but test the water and cast your mind back over events around the pond during the last few weeks and you might find your answer.

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Our pond fish were dying at a rate of three or four a week over about a three week period before we discovered a large toad who was swiftly removed to a neighbour's wildlife pond. All frog/toad access to the fish pond was removed. No fish deaths since and we firmly believe that we identified the culprit...
Deb - 27-Mar-21 @ 5:04 PM
Hi, I have a 1000 gallon pond, well established, well filtered and aerated. Trouble free for years until the last two weeks. Goldfish are dying daily and showing signs of swim bladder problems. Did a whole pond antifungal and bacterial treatment. One week later did a 25% water change. Isolated fish showing symptoms and treated with swim bladder medication. I have been checking water quality every two days and ph, nitrite, nitrate, ammonia etc are all good. The really strange thing is the koi in the pond are all ok with no symptoms or deaths. Any ideas what could be killing the goldfish? The only trigger I can think of is a cold snap and the pond iced over, however I kept an area clear with aeration so would assume gas exchange continued without too much change. Please help, I am stuck!
Jason - 12-Mar-21 @ 8:02 AM
We have had a large pond for year and have recently lost4 f all a decent size now tonight I have found our 10lb koi diedno signs of diseaseany suggestionspond is well oxygenated
Gail - 30-Nov-20 @ 10:11 PM
Hi iv had a garden pond for years the normal run of the mill fish goldfish, koi , roach gudgeon, all of sudden I've lost all my gudgeon family and now I've started losing my big roach no signs of damage all the others are so far ok. Thought it may be the fireworks Any on any ideas pleased
Bulli1 - 15-Nov-20 @ 12:28 PM
We’ve lost more fish this year too. We inherited the pond 5 years ago when we bought the house but have educated ourselves on how to care for it. Each year we have usually lost between 3 and 5 after spawning. This year we have continued to lose throughout the year, it’s only ever one at a time and the others are all fine - until we find another one dead. No injury or disease evident on them.
Fifi - 5-Nov-20 @ 11:45 AM
Although my fish have lived in my pond and reproduced here successfully for 12 years, all of a sudden about 15 of the largest fish and on small died, but what has me confused, is that 2 small ones were still swimming at the bottom. Wouldn't they be affected in the same way as the large fish (goldfish)?
Dianne - 30-Sep-20 @ 7:10 PM
I'm really annoyed another of my fish is dead,opening bag of food the Fish food smelt really strong
Sads - 26-Sep-20 @ 5:37 PM
What's the name of brand of food your feeding the Fish with? 7 off my orange goldfish are now dead!
Sads - 22-Sep-20 @ 10:36 PM
I added pond salt to my fishpond approximately a month ago. I calculated the pond gallons, had another person check behind me and followed the exact directions, but I did cut back on the salt. My fish have been active, jumping, and ready to eat bubbling and swarming the surface for food.Just as they always had before.No sign of a problem.I went out this afternoon and all of my 40 fish were dead .Large and small fish.Why?If the salt was to much, would they have lived a month and continued to act normal?Would they have died all at once or a few at a time. I did recently purchased a new bag of food.Please help me understand. Thank you
Sher - 10-Sep-20 @ 8:58 PM
I had 4 koi die within about 2 weeks of each other, thought it was down to spawning for the 2 females and the males being brutal. Then the 2 males died. No obvious signs of damage but they were gulping around the surface a lot. Couldn't have been lack of oxygen as I had 2 air stones pumping 70 litres per minute in there. Anyway long story short I think it was a chemical the farmer had used to kill his crop ready for the next crop. There more I think about it there was a chemical sheen on the surfaceand I put this down to spawning and protein at the time as there was foam present too. My advice is if you live near any fields or farms check when your farmer will be spraying his crops and cover your pond before during and a little bit after its been done. Best of luck to you all
matt - 30-Aug-20 @ 5:10 PM
Same here... Woke up yesterday morning 2 wore gone and one sick... The only thing we could think of was that something toxic got in the water... We haven't made any changes in food water or anything lately so we took most of the water out and replaced it and they seemed to be better... The one sick has been separated and we are struggling with it... This is so much more worrying as they weren't the smallest ones.
Diana - 21-Aug-20 @ 8:07 PM
I have kept my koi for over 30 years no problem but the last week has been a disaster I lost a monster 25lb koi followed now by 3 small ones and I have another one very stressed he is black and white but the white is showing raised blood cells and veins also he is always on the surface with head facing down It’s very upsetting my pond is 2600 gallons with huge filtration and 4 air stones plus a Venturi return and a 2 inch return from a trickle tower My water has tested perfect but the temp did rise to 29c recently but now it’s 22c I am at my wits end and very upset and my wife is in tears
Hurricane - 17-Aug-20 @ 6:17 PM
We've had a Koi pond for almost 30 year's with no problems until this year. So far we've had sudden deaths of 2 very large and established koi shortly followed by several smaller one's. I bought 3 replacements and already one of those has died..no symptoms just lopsided floating then sinking to the bottom and then dead on the top. My Pond guy is equally baffled...the water samples each week are fine but every few days another one has to be hoiked out. We live in Bournemouth any other locals having same issues?
Gaz - 16-Aug-20 @ 12:59 PM
Hi need some advice.My 20 year 1500 gal pond has good water quality (test 2-3 times per week).Good filtration etc.I keep 5 Orfe, a couple of bred in the pond around 15 years old Goldfish and Koi.The Koi ranged in age from 3 to 15 years old.My pond was healthy.I added 10 x 4" koi 2 months ago.Within 2 weeks I had lost a couple of the 4".I wasn't to worried as no sign of disease but more closely monitored and prepared to treat as needed.By 4 weeks I'd lost all 10.I'd treated for broad spectrum bacterial infection but didnt stop the dieing.Still no visible sign of disease.Now the real heart break begins one by one my adult Koi are dieing.They stop eating, activity levels fall off and after a day or two they die.There's no rot, fins are intact, no weight loss etc.No sores, or any sign of a parasite, no excess mucus etc.The Orfe and Goldfish remain active.So far I've done all the normal stuff, water quality, filter check, adequate water flow, filter cleaning as required, anti bacterial, white spot treatment, just in case.I've treated for parasites.As medicine brand is often a question people discuss for the potential parasite/fluke, it wasn't Kusuri or NT Labs.Albeit I now have both.It was a more general parasite treatment active ingredients Formaldehyde, Methylene Nlue, Malachite Green Oxalate.So before I try one Fluke treatment now that all previous treatments have had time to subside.Does anyone have any other ideas as to what is killing only the koi, especially as dieing is so spread out.
Scotpond - 7-Aug-20 @ 1:13 PM
My fish are dying, too.No reason.It started when I purchased a new bag of food (same brand).May I ask what brand food y'all are feeding your pond fish?
Tiffany - 31-Jul-20 @ 8:17 PM
I recently had my fish pond deep cleaned. The fish were transferred to a holding tank filled with the dirty/silted water from the pond. They made a very good job of cleaning the pond but before the pond was filled with De-Chlorinated mains water, about 20% of the dirty/silted water from the holding tank was pumped back into the pond, the fish were then returned before filling the rest of the pond with De-Chlorinated mains water. When I mentioned about returning the dirty/silted water to the pond I was told the fish needed it or they could be shocked with fresh water. Is this correct? as the water still has suspended silt gradually falling to the bottom of the pond. The water is not nice and clear as I had expected.
Littlebarney - 23-Jul-20 @ 8:38 PM
I have just recently changed my uv and cleaned my filter but my pond has gone greener than before I had my water checked and it is 7.4 which they said was fine But my fish don’t seemed to be feeding and staying on the bottom I have lost about 8 fish in the last 3weeks any ideas thanks rob
Shaft - 14-Jul-20 @ 9:36 PM
i have lost 10 koi in the last 2 months. I find them dead in my skimmer box at least once a week . my pond is 1100 gal with a 18 foot desending steam and waterfalls to pond,,,use a 3200 gal pump from the skimmer box up to the upper filter box.. they come to me and eat fine,,,i also have 4 large airation stones spread around pond running 24/7....water tests perfect on all levels including ph.....some fish seem to be congregating under the waterfall but no apparent issues,,,but then i find 1 or 2 dead in the skimmer box,,,,baffling me.....have had my pond 10 yrs and have never had a problem like this before......
pondguy - 3-Jul-20 @ 9:22 PM
So 2 of my fish are acting very strange. They will swim around like mad in a circle and then jump out of the water a couple times and then they just freeze and sink to the bottom. Is there anything I can do? I isolated them and have a air stone in there iso tank but I’m not sure what I’m doing wrong.
Pond lady - 26-Jun-20 @ 8:16 PM
I had a near total pond collapse one year during a hot summer despite having a waterfall, fountain and filter pump going 24 hours per day. Turned out I was topping up/cooling down the pond with butt water which had far too many nutrients in it from the fallen and decaying leaves in the guttering!
Woofer - 7-Jun-20 @ 12:03 PM
We have been losing our fish one a day for a year nownearly all the smaller fish but now our larger koi. Water tests all okhad skin scrapes tested at local koi farmall test show finewe have sturlets in pond they are fine.We have tried everything it is heartbreaking. My largest fish is now dying. As soon as we remove a dead fishanother one becomes lethargic at the top of pond then goes to the bottom lays on its side and dies. We are down to a handful of fish. Both myself and my partner have always kept ponds and never seen anything like this.Staff at koi farm have tried to help but cannot see the problem
Jacqui343 - 26-May-20 @ 7:35 PM
I have a garden pond about 15 cubic m of water,120l/minuteaeration,600 liters of k1 in 2 ibc home made filters,whith another 220 litter filter and3 aditional filter boxes whith japan mate .In pond i had for 3 years now 12 big carps,and 700 babies from past year.I use ground water from 15 metter depth.Never had more then maximum 10 little fish dyng per year.Last week all the small fish start dyng,about 50 per day..after 2 days i change 70 procent of the watter ,put 25 kg of no iodized salt,but almost all little fish are dead now,and i big female carp .I have many plants,my watter hyacinth ware affected by salt,and i put them to recover out of pond.The pond is in Bucharest,Romania.All the smal fish are dead now,and they have no injuries,no red spots,nothing,look like same mysterios poison killed them.The planes are sparyng heavy chemtrail,the water teat looks fine,but can not measure aluminium oxyde and barium who are sprayed upon the city.I am crazy,or what ?What can be the reason for fish dyng,please help me ,all tests are fine,no amonia or nitrite,oxygen ok,temperature of water 20 degrees celsius.Help me please whith same advice.
Dany - 15-May-20 @ 6:55 AM
I am currently losing a small koi every day. The larger ones seem ok but it’s not great I have lost 20 or so now. I have tested the water and it all seems with normal ranges the water has added oxygen and also oxygenating plants. I have seen on some sights that I maybe should change 10% of water per week making sure I remove chlorine and ammonia.Do you have any suggestions
Dave - 8-May-20 @ 7:16 AM
Lost a fish 2 weeks ago now and just looked in pond 2 more dying they no spots on them
Dons - 13-Feb-20 @ 3:40 PM
Hi just changed the water 2/3rds of the pond this January because fish were gasping for air and I’m starting to lose now my young fish and some older fish about 4 to 7 fish a day pond is about 6ft x 15ft x 4ft deep just used tap water which iv used in the past any suggestions please thanks Dieter
dieterlost - 13-Feb-20 @ 1:28 PM
Hi just changed the water 2/3rds of the pond this January because fish were gasping for air and I’m starting to lose now my young fish and some older fish about 4 to 7 fish a day pond is about 6ft x 15ft x 4ft deep just used tap water which iv used in the past any suggestions please thanks Dieter
Dieterlost - 25-Jan-20 @ 4:33 PM
We have a pond in this house a year we had ,had nothing but problems we brought our fish from our other pond but now everytime we change the water, or weather rain our loi are dying we done ph things we feed well they are our babies and they keep dying can someone please us keep them Sit
Liynds - 12-Oct-19 @ 10:32 PM
I have a large natural pond, 35x24 yards average depth 3 feet. Large gravel filter the size of half a tennis court. Water great, all tests show perfect O2 levels and water meets all other tests. Seven years with one or two losses per year, about 500 fish all but two are carp. 18inches to this years spry.. 300 died overnight. No berries, no crop spraying, no run off, no nothing except a hot day, for England 75 and a cold night 37 with heavy rain that night. Lost another 50 the next night and so on, only a few left now. No marks, no parasites, no where spot, no yellow gill. HELP!
Guv - 18-Jun-19 @ 5:20 PM
Would it do any harm to increase the size of the filter boxnow my fish are a lot bigger. The present filter is not keeping the water as clear or blanket weed free as it use to?
Liz - 3-May-19 @ 8:16 AM
We’ve had a garden pond for the last 30 years plus but this year have lost many, many fish. For no apparent reason. Had water professionally tested,filters, air pumpsinfra red lights etc. Never had a problem before. Please help.Today. Fish papered to have blood under its gills! There are a lot of big fish but also quite young fish too.At a loss to know what to do.
Mamma - 25-Mar-19 @ 1:54 PM
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