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What Could be the Reason for all the Fish Dying?

By: Dr Gareth Evans - Updated: 14 Jul 2020 | 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)

A.

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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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
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
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