Thread: Fish Death - Help needed please!

The pH looks quite low considering the high GH. Do you have a KH reading?

The main culprits for any situation like this will normally be down to water or parasites.

Both can extend into a lot of different possibilities, ie. Oxygen levels, pH fluctuations, secondary bacterial infections following parasite attack etc.

It is really difficult to determine much from the photos, and the fish doesn't look in a terrible state, but was obviously suffering some kind of attack, bacterial or parasitic.

The worm on the microscope isn't a fluke, and is most likely not parasitic.

The only thing that really stands out to me is the swollen eyes - were they like this before the fish died? Are any of the remaining fish suffering with swollen eyes, sunken eyes, or raised scales?

cheers g mac

I've tested for chlorine and it didn't show up but just in case i've put some de-chlorinator in

Koi flashing goes down to the 2 things listed in my previous post - Water or Parasites.

KH is quite important (more so than GH) because it is the buffer that helps maintain a stable pH.

KH gets depleted over time and can only be replenished by changing water, or by adding bi-carb if your source water is already depleted. When KH gets to zero the pH can crash causing Koi deaths with no obvious visible cause.

I'm only pushing the KH aspect because with a pH of 6.9 (or even lower as you stated) it could be a factor.

If you've never tested KH before it would be worth comparing from the tap and from the pond to give you an idea where your pond sits in relation to the fresh water that you add to it.

If all those things check out ok, then parasites are the next thing to look for - which means more scrapes and microscope work till you get to the bottom of it.

If you have any other koi showing similar sores or inuries you can pull them out and treat with tamodine to reduce risk of secondary bacterial infection while you continue investigating the route cause.

The Colombo test kits are pretty good, I've used them before alongside the API test kits. I've now moved to the JBL pro aqua test kits which are expensive, but certainly the best I've used.

I have no clue where i've put the KH vial, so i've ordered another kit, can't go for the JBL one as i'm skint at the mo haha

It's coming Friday so i'll post updated readings including a KH result on Friday.
I won't be able to compare the pond to tap, as i just did a fairly big water change yesterday so i'd say it'll be very similar now.

I'll keep scraping any fish i see flashing and anything dodgy i'll post here

Thanks for all of your help both, i really appreciate it

Update:
On 31st July i did a scrape on all fish, found flukes on 1 so i put in fluke-ex to clear that.
The fish all seemed happy and were fine for a couple of days. Today (07/08/2025) i saw one flashing, scraped. Flukes again.

There are seagulls that keep going into the pond trying to get the koi food, could this be a cause of them? I'm trying to figure out a way to get rid of them ,they just land on the net and get their feet/stomach to touch the water.

The water test kit has gone MIA in the post so still waiting on a fresh one of those to do the water test properly.

Is it too early to do another dose of fluke-ex or another product which you could recommend? It says re-dose after 3 weeks but that means they're going to have flukes thriving for another 2 weeks in the pond and i don't want to lose another fish.
And also would you recommend putting chloramine T in the day before to soften their mucus layer before the fluke treatment?

Thanks for your help

So it's been a while since i last posted.

I got the test kit, i've tested it a few times on random days and random times and the results are consistent.

The fish are still flashing, although no more deaths so far.
I cleaned out my filter and also cleaned the bottom of the pond.
I've done another 2 treatments of fluke-ex since my last post.
I've ran scrapes on three fish today that i saw flashing and found gill fluke on 1, all other scrapes are clean. Struggling to get these completely gone...

The water parameters are as follows:

PH - 7.5
Ammonia - 0
Nitrite - 0
Nitrate - ~5ppm
Phosphate - 1.5 ppm
KH - 8
GH > 1

There looks to be a small leak in the liner but that's only appeared over the last few days. Nothing i can do about this until i save up enough to get the pond fiberglassed sadly.

Does anyone have any idea on why my koi are still unhappy, is it still these flukes as i found one?

Attached pictures of the fluke i found and also pics of the koi just so this post isn't all doom and gloom

Thank you,

Originally Posted by Alburglar

Are you stripping the mucous before using the fluke treatment? You may have to try another treatment.

Thanks for your response -

I have tried 4 lots of fluke-ex & 2 lots of fluke M so far. I used fluke solve plus last year but that didn't do anything.

I've been stripping the mucus with chloramine T but the fish are flashing and going up for air (there's plenty of aeration in pond) so i believe it's the gill flukes so i need to give another course but my water temp is below 12 degrees which it says on the chlomarine T pack is the lowest it should go, so a little unsure on what to use now... but i need to use something!

i would turn my uv on and do a big water change to get rid of the CT and any residual treatments ,salt and use SV .The gulls landing on the pond are probably stressing the hell out of your fish because they think they are under attack,supressing their immune systems.Just my opinion.Thats what i would do,and situations like this remind me of two things
A small elecro heater would gently lift the temps to help
If you put your location you may have a forum member in the same town who could call in and help

Originally Posted by big h

i would turn my uv on and do a big water change to get rid of the CT and any residual treatments ,salt and use SV .The gulls landing on the pond are probably stressing the hell out of your fish because they think they are under attack,supressing their immune systems.Just my opinion.Thats what i would do,and situations like this remind me of two things
A small elecro heater would gently lift the temps to help
If you put your location you may have a forum member in the same town who could call in and help

Thanks for your reply!

Water change + uv has been done a few days ago.
Fortunately seen no gulls on the pond for a week or so now
Is SV supaverm? I don't have any sadly, I own no livestock haha.

Im a bit out of the way so i doubt anyone would be near - lake district. Barrow-in-Furness.

Ive ran out of salt and the fish need treating so im going to try the PP route mentioned earlier and see how it goes.

Originally Posted by g mac

Sounds like you and your fish are having a rough time of it. That's a lot of treatments administered this season.
It seems strange that it has not been effective. Perhaps double check your dosage/pond volume etc.

Before using PP make sure everything is as clean as possible.

PP is quite harsh and given your fish are weakened and stressed, they will require careful monitoring during the four hours of treatment. Make sure to have enough Hydrogen Peroxide on hand, in case you need to neutralise the PP in a hurry.

I would do a few scrapes the day after the PP and then decide on an appropriate treatment.


Good luck.

Cheers g mac.
Its been a very rough year for the fish!
Im going to clean the filter thoroughly now. I think the way the liners been put in there's a lot of folds so im getting quotes to see if I can get it fiberglassed with a bottom drain so there's nowhere for anything to hide.

My dosage and pond volume are correct, I will double check but I follow the instructions carefully

I'll do PP but will I not need fluke treatment too? They look like Gill flukes so they'll be laying eggs that need treating will they not?

Thanks