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What Could be the Reason for all the Fish Dying?

By: Dr Gareth Evans - Updated: 12 Oct 2019 | 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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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
We are so so sad This morning all our koi have died. My daughter is so upset! I just can’t understand what’s happened. They were so well looked after. ??????
Bhav - 3-Nov-18 @ 10:43 AM
Hi I have had my pond for over 10 years, natural fish deaths have occurred over this time but this year I have lost 7 Koi, I am lost for a solution, water tests all ok , filter good, good oxygen, the symptoms ... stop feeding, become listless, all gathering at one end of the pond pretty motionless, began in July. There was no obvious signs of a problem, a little flashing , so i treated for parasites and flukes, but still the deaths continued, one Koi left , the largest, can't do much now , hope it survives the winter but I doubt it
Redmanna - 3-Oct-18 @ 9:55 AM
We have had fish for years in our pond and many babies. Pond is filtered, oxygenating plants, lillies etc. We are losing a fish a day and cant see any marks on them. Tested water and within range. Just want to know what else we can do to protect the other fishes. Looking on here at all the posts of fish dying it doesn’t look like there is an easy answer
Dahlialodge - 19-Sep-18 @ 9:59 AM
Had our fish for years no problem had we had many babies. They all seem to be growing beautifully, lively, now we are having one fish dead every day. So upsetting. Pond has oxygenating plants, filter, lilies for shade. Water tested and levels are within range. Looking at all the posts on here seems are problem everywhere. Just need to know why to save the remaining fish
Dahlialodge - 19-Sep-18 @ 9:54 AM
Hi I to have lost 7 fish over the last 2weeks and reading the rest of the Comments I've come to the conclusion that it's the tap water my friend would only Refill his pond on certain days because he said that the chemicals that thay add Are very dangerous to koi .
Andy b - 12-Aug-18 @ 1:14 PM
I've lossed 17 fish in two weeks, including 20year old koi. Water test fine. Got pump, filter box &2 airiaters. Fish I have left are on the bottom, listless with find closed. Sad to see but don't no what to do for them. Any ideas anyone?
Ginger - 30-Jul-18 @ 4:40 PM
Hi There hope you can help I have a pond 24m x20m x 3 to 6ft deep and in the last 2-3 days have lost quite a few fish ranging from 20lb to 1lb. In the hot weather they seemed to be fine and still feeding. But since we had a large down poor of rain and a considerable drop in temperature they seem to be slowley dying. Do you think this is the cause as they look healthy with no obvious marks or desise
silly - 30-Jul-18 @ 12:55 PM
Have a wildlife pond in my garden have 3 spin sticklebacks and other wildlife today found two sticklebacks dead not sure why, could it be the water as the hot water continues have had to use tap water. Please advise Clare
Pippa - 7-Jul-18 @ 8:06 PM
I have a 10 year old pond approx. 10,000 gallons, 18 x 35, 4 feet deep in the middle. I have been losing 1 -2 fish a day for the last 2 weeks and don't know why. The fish are 5 lbs to 40 lbs. Great filtration. Bio bead filter. Solid separater. UV light with new bulb annually. Waterfall. Couple plants. Fish lying still at edges of the pond, not eating over the past 2 weeks. Surfaced scum they tell me is from spawning for about a month, yet my fish are dying. No sign of any problems on the fish. Pond is extremely clear. Half shade and half sun yet fish still dying. 0.13 salt level. No traces of nitrate at all. PH 7.5. Haven't lost a fish in 10 years. Pond is maintained and enjoyed every day. I don't use chemicals or fertilizers in my yard. Appreciate your advice.
Jack - 16-Jun-18 @ 9:46 PM
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
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