Home > Ask Our Experts > What Could be the Reason for all the Fish Dying?

What Could be the Reason for all the Fish Dying?

By: Dr Gareth Evans - Updated: 2 Aug 2017 | 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.

You might also like...
Share Your Story, Join the Discussion or Seek Advice..
[Add a Comment]
if you have water lilies and they are overcrowded they are also like a jungle below the water line fish can become entangled and get stressed also it can cause gas to be trapped below water and pockets form. if this is the case then try thinning the plants below the water line with pruning clippers and open up some of the surface and get rid of the jungle below. if not look at the airaintion of the pond as to much exposed to sunlight will deplete oxygen levels again deadly for the fish
kev1 - 2-Aug-17 @ 6:05 AM
Colin Pearce - Your Question:
My 2 brothers and I have ponds and this last week we have all had fish loss. Ponds in various locations, no signs just dead, my brother cleaned his filter and it was quite clean and no smell as it normally has this had been running a couple of months ? I am wondering if the water board has added something over this last week or so because of the hot weather ?

Our Response:
Contact your environment agency or canals and rivers trust and ask them if they are aware of anything, especially if another person's pond has been affected.
PondExpert - 26-Jun-17 @ 2:06 PM
P.S. 2 ponds have been established for 20 years no problems before and 1 pond 12 month old
Colin Pearce - 23-Jun-17 @ 6:38 PM
my 2 brothers and I have ponds and this last week we have all had fish loss. Ponds in various locations, no signs just dead, my brother cleaned his filter and it was quite clean and no smell as it normally has this had been running a couple of months ? I am wondering if the water board has added something over this last week or so because of the hot weather ?
Colin Pearce - 23-Jun-17 @ 6:36 PM
We have had a pond for years and have had no problems until about 6 months ago,we bought several new fish and they have now all died ,even the ones we all ready had in the pond have all died,we have cleaned the pond out which has not helped.we have plenty of frogs and tadpoles.Any idea what the problem can be.
Spider - 16-Apr-17 @ 4:39 PM
Hello, I have a natural pond 10m long x 7m wide, 1 1/2m deep with approx 90.000lt water taken from the local river. I have for sometime a trout and a salmon-trout of about 1kg each. Last week, I have bought at a acquacoltura farm, 4 young tench and 5 trouts of about 150g each. The problem is that two of the tench seems to have developed some bubbles on their skin and they can be seen still, among the aquatic water plants just below water level. The rest of them, I have yet to see them because the water at the moment is rather turbid "not clear", because of a recent heavy rain falls. Your help would be apreciated... many thanks
N/A - 29-Mar-17 @ 6:28 PM
Hi, I have an 8ft x 8ft pond with 10 adult Sticklebacks and 10 Rosy Red Minnows. RecentlyI added some Aquaplancton to get rid of the algae and blanket weed, but now I cannot see any adult fish at all, I haven't seen any dead fish. My Sticklebacks have had babies and they are still there, but what could have happened to all the adult fish?
Martin - 6-Aug-16 @ 9:52 AM
hi last year i cleaned my aunties pond out it was very neglected i removed a good three inch of sludge from the bottom all was going fine until this summer when the fish started dying the pond is around 2000 gallons and filtered by a hozelock easyclean filter
craigmason71 - 29-Jul-16 @ 2:14 PM
zoyland - Your Question:
We have just doubled our pond size, were very careful to use all the right water cleansers with the tap water when we refilled, we have pump, waterfall and u.v.filter ,have reeds, lilly and water iris, but still one by one my eight new fish have died! someone help please!

Our Response:
Were the fish originally in your old pond? Did the water temperature changed dramatically when you build the new pond etc? Sorry we don't have the perfect answer to this as there are so many reasons why pond fish can die.
PondExpert - 7-Jun-16 @ 12:13 PM
we have just doubled our pond size, were very careful to use all the right water cleansers with the tap water when we refilled, we have pump, waterfall and u.v.filter ,have reeds, lilly and water iris, but still one by one my eight new fish have died! someone help please!
zoyland - 5-Jun-16 @ 3:30 PM
My fish have been dying 1 or 2 a day for a week. I did clean the pond out a couple of weeks ago of some pond weed which I do every year as it accumulates if not kept under control. Could this be the cause? Would appreciate any help from anyone. Thank you.
Glynne - 30-Aug-15 @ 5:33 PM
Spring cleaned my 800 ltr pond with borrowed pond vac, thinned out elodea plants. Cleaned filter (submerged with integral uv) and tested water for all the usual suspects. Oxygen level 12ppm, nitrate and nitrite both 0, ammonia 0 ph nuetral. Put 2 new 2 year old goldfish in with the 6 10 year olds and the 2 tench. Within 2 days all the older fish developed every fungal desease ging and pop eye, cloudy eye etc. All spent about a week on the bottom not feeding and lethargic. I treated the pond with Tetra medifin and introduced additional aeration. The fish became more active but despite 2 Medifin treatments over 10 days the deseases rage on and I have lost all but the biggest of the older golfish and 1 of the tench. The 2 younger and newer fish are the only ones not looking too bad and are still active and feeding. Any ideas as to what might be going on? Also what do I do to make the pond safe to restock eventually.
Gray - 13-Jul-15 @ 4:19 PM
Hi someone stole our water lily and cut the net over our pond about 2 weeks ago since then we have had 11 fish die some floating some of the bottom of pond we have a waterfall and 2 pumps we still have one lilyand plenty of other plants the water is clear what can cause there death
Jules - 9-Jul-15 @ 10:21 PM
Over the past few day we have lost about 12 fish from our pond.We have had the water tested which was ok and the water is crystal clear.We have a filter, pump, waterfall and fountain plus a lot of oxygenating weed.We are at a loss to know what is wrong. Help!,
Bluebell - 30-Jun-15 @ 6:53 PM
I have a pond 20ft x10ft.capacity of water is 18000 ltd.In past There are about 100 fish most of them are gold fish and 1 koi.I past 3 days 15 fishes have died.I introduced barley straws and elodea plants last week but I tookthem out after fish started dying.I cleaned the pond2 days ago and changed 70 percent of water and also added dechlorinater and next day added blacket weed treatment .I also added pond aeration set and added extra pump to give a fall for extra oxygen.Second pump filters water through bio filter which I also clean every week. I had water tested before I and after I cleaned the pond but i am told there was nothing wrong with both the water.I found two fish dead this morning.I don't know what more to do can you please help me .
Ken - 20-Jun-15 @ 9:30 AM
Hi just recently I changed the water of my fish pond. I also added weed cleaner into the pond the first time after I changed the water. But since then two fish have died who didn't look unwell.My fish pond is new only about 16 months since it was first build so the filter etc is all brand new. I am very worried for the rest of the fish and I need some good advice. So please reply to my enquiry as soon as you can. Thanks
hussain - 6-Jun-15 @ 11:14 PM
January 2015 , had pond fish for years, never had any problems, this time of year fish are usually at the bottom, but last few days they are at the top dead , when you touch them they feel frozen, butit is sunny. am unsure if I have any fish left, I think maybe its geo engineering (Chemtrails)
hillie - 18-Jan-15 @ 1:34 PM
Hi I have had 12 roach and 6 goldfish in my pond ( 7ft x 3ft x 2ft deep ) for 4 years. They have always been lively and healthy, and have grown well. Over the last 2 weeks all the roach have died but the goldfish are ok. so upsetting, any ideas thanks
geoffo - 26-Oct-14 @ 4:21 PM
My koi are all dying. We have lost 8 since the beginning of the year, most since the really warm spell in July. Just lost my favourite fish, 18+ inches! Had him since he was 2inches. Lovely fish. No visible signs on any of them. Water tests all good. Plenty of filtration and oxygen. Local koi dealer reckons koi herpes, so all of them will die. Not affecting sturgeon or our one goldfish. We have over the last two years put9 small Japanese koi in the pond from reputable dealer, thanks to heron only 4 left, but one of those died yesterday. We bought a filter system from e-bay earlier in the year, this was all dry and cleaned, we understand herpes can't live if it dries, so hope it is not that. Understand Japanese koi worst thing to buy, as these are usually the problem. Is there any cure?
Peejay - 12-Sep-14 @ 7:11 PM
I have lost 6fish in the last few daysthey all seem to have damage to they side like they have been attcted is this potable ? I still have 4fish left
Sam - 28-Aug-14 @ 10:57 AM
We had twenty kio and one goldfish and four cat fish in our pond. they were are doing fine. The last week we have lost all but the goldfish. we dont know what went wrong. do you have to have chemicals in the water are what do you have to do to keep your water clear and keep your fish alive?
BUCK - 21-Aug-14 @ 2:48 AM
@VinceUK. Warm temperatures often cause problems with pond water but sudden changes can also be a shock to the fish - if you want to change the water, just change half of it slowly. Check also whether you have too many fish in the pond or whether there is uneaten fishfood starting to rot. Plants like water lilies will provide shade and a submersible pump will also help to keep the temperature down while a filter will help keep the water clear.
PondExpert - 4-Aug-14 @ 12:46 PM
hi I've been keeping pond fish for a long time now fish in pond are carp roach and tench , since the hot weather I've had 24 roach die and one koi in a mater of 2 days. all the other koi are fine except the roach , the water is green so im thinking as worm water doesn't hold as much oxygen as cold water I think this is the problem so is the green water so im changing almost all the water to see if this stops it .
vinceuk - 31-Jul-14 @ 10:07 PM
Hi, In the past week I have found between 1-4 dead fish in my pond each day.I have checked the water quality and all appears Ok.I have kept the pump on 24/7 and have an added solar pump to provide extra movement.I have several very large Koi which thankfully have not succumbed it has mainly affected the gold fish and one golden Orfe.All fish appear perfect, just dead. Its very disheartening to see perfect fish die in front of you and would really appreciate any ideas.
Ianhelp - 16-Jul-14 @ 5:35 PM
Speedy...we don't really have enough information from you but as we have been getting so many questions in about pond fish dying, we will be providing a new feature with frequently asked questions on this soon!
PondExpert - 2-Jul-14 @ 11:20 AM
pond fish dying day after day all look fine no obvious disease water good, pump on. only clue is a history of herpes in the area.help.
speedy - 30-Jun-14 @ 3:48 PM
My pond is 7 years old After having to enter the pond I am losing Fish ..so far 12 ..I thought I could have stirred the bottom up and effected the oxygen level..added another pump no help lost three last night. No chlorine present the pH is a little low to neutral. All of the fish are the oldest one in the pond no young ones. Nothing has changed from the past years ..I have gotten in the pond before but this has never happened. They are at the top gasping ..but I have seen that before. The fish are eating well ...please help..Oh my pond is 18 by 12 and 4.5 deep in the middle with a 2 foot shelf.....
kytarheel - 26-Jun-14 @ 3:05 PM
@Wobbly. Without much detail there is not much we can say but here are a few things to think about: (1) Are the other fish in the water all OK? If so then it's unlikely to be a problem with the water, but test the water if that's the only fish. (2) Could the fish be pregnant/about to spawn? Or egg bound? (3) Could it be dropsy? You would find your fish was not swimming normally if this were the case though, but may be something to watch (4) A genetic stunted growth syndrome where fish grow outwards instead of lengthways and hence become bloated? (5) Other infections such as a worm, bacterial or intestinal disease? If so you'd be advised to check out suitabe medications at your local aquatics supplier. A few things to think about there but you're better off seeking expert help from someone who can see the fish first hand. Keep us posted.
PondExpert - 19-Jun-14 @ 10:46 AM
I have a fish in my pond that has got extremely bloated, looks like its about to explode, but is still swimming about normaly,what could the cause of this be please
wobbly - 18-Jun-14 @ 6:33 PM
I have had my current pond for 5 years.It is 1300 gallons and just under 6 feet deep at its deepest point. In the last 2 years fish keep on dying, older fish at first died, many where over 15 years old (transferred originally from a smaller pond).the other new small fish I have introduced have all died.I have tested the water quality several times at it is OK..?..the [plants in the pond are thriving.I have a UV filter which keeps the pond clean.Two weeks ago after having the pond empty since January, the water was tested, it was fine, I bought 10 small goldfish to try.after about 4 days the fish stated looking lethargic, day 6 or 7 I could scoop them out with my hand easily, now some are gasping at the surface and look as though they are struggling and in discomfort.there are no visible signs at all of parasites, spot, cloudy eyes or skin, they look healthy fish.What they have not been doing since introduction is eating food, and I bought fresh food again when buying the fish.I have overcome some diseases within the pond and my tropical fish tanks over years, but this has me confused as I have done everything I think I should be doing?any help appreciated.
Ian - 10-Jun-14 @ 1:02 PM
Share Your Story, Join the Discussion or Seek Advice...
(never shown)
(never shown)
(never shown)
(never shown)
Enter word:
Latest Comments
Further Reading...
Our Most Popular...
Add to my Yahoo!
Add to Google
Stumble this
Add to Twitter
Add To Facebook
RSS feed
You should seek independent professional advice before acting upon any information on the PondExpert website. Please read our Disclaimer.