Hi all,

New member here, so hello to one and all.

I'm turning to you guys and gals for emergency help please! Not a good starting point I'm afraid as I'm already starting to loose the fish I re-introduced to my pond last week!

History: 4 years ago I had a fibre-glass pond built 3m x 2m x 1m deep. Have only filled it up once and never cleaned it out. (I was trying to adopt a natural ecology balance as my thought process was that lakes do not get cleaned out yearly) I had a pump & filter system in place which has been used little as my initial readings on keeping Koi were that a clean pond is not the best environment and it was good to even introduce mud/clay into the pond which I did. Plants have all become well established including bull-rushes, also water snails and water boatmen are all in there. I introduced Koi into the pond and lost them initially and do not know why. After some time I introduced more Koi, up to 13 in fact. Last year they even performed at mating and I had eggs ! None hatched. I lost my big white Koi first of all back in early summer but couldn't find him and still haven't (heron is doubtful, a cat did take two or three though & left them on side of patio, heads missing, yuk!) I noticed one fish had a diseased fin and should have taken him out straight away but didn't as thought it was damage by other carp. They all eventually died by late summer 2010. My initial thoughts were that they were suffering a lack of oxygen (I had lots of oxygenating weed in place) as I could pick them up by hand and were drowsy or just sick? The frogs done very well last year. When the ice melted this year I found all the frogs dead. I was however really glad when I spotted the frog spawn a few weeks ago and since then I've spotted 3 large frogs in the pond. Last week I introduced 5 goldfish as a trial and alas I fished one out today who was still alive but on his last fins. Others seem drowsy and I could pick one up by hand. There's an oily slick on top of the pond which seems to come and go. I suspect some kind of poisonous weed or plant that I've introduced but am completely stumped. I've turned the pump and filter on to get the oxygen levels flowing. Here endeth this monologue and hope someone can shed some light on the cause of this disaster. Thanks

Hello and welcome to the site :)

You say the filter has seen little use? The filter will need to be on 24/7 as it's main job is not to keep the water clear but to grow friendly bacteria that consume the ammonia and nitrite in the water. Fish produce ammonia all the time and it's highly toxic to them.
The filter also creates water movement which aids in oxygenating the water far more than a few weeds can and should break up that oily surface film.


It takes 6-8 weeks to build a colony of bacteria in a filter that can help detoxify the water. So turning the filter on now isn't going to be enough to save the day. Can you test the water for ammonia, I think this should be your first step. If you find there is ammonia in the water products like AmQuel(used sparingly) and partial water changes will be required until the filter catches up.

A pond expert will be along soon and I'm sure they'll be able to offer more detailed advice - I assume the fiberglass was professionally cured/sealed to make it safe for the fish?

Many thanks EagleC,

Blimey what an amateur I am. I didn't use the filter & pump much because it never cleared the water and always seemed murky. I have never cleaned out the filter (its a pressure filter) as didn't want to destroy the bacteria that had built up inside. Do you think I should clean it out? And do you think I should empty say half the pond water and top up with fresh? I shall check the ammonia levels with a kit I'll get tomorrow. If it is ammonia then it must stick around for months coz there's not been any fish in the pond for around 7-8 months if not longer.
As far as I can remember the guy sealed the glass pond though there was a small hole in the bottom which seems to have sealed itself.

Appreciate your reply

Hello Waterbeetle, welcome to the best part of the forums.

It seems that if there is something you should not do for a koi pond, sorry, but through wrong choices, you have done it, but let us turn it around for you.

First of all I would suggest that you completely empty the pond, and give it a good clean (Hose, dustpan and brush, no chemicals, or you can use a pressure washer if you have one as it’s a GRP pond)
The reason being, as EagleC said the filter should be running 24/7 (More about filters later)

Oxygenating plants do indeed give out oxygen by day, but did you know they take it in at night? I would therefore suggest that they go on the compost pile, along with all the other plants, the other reason being that healthy koi will eat plants, turn over any soil they are in which in turn clouds the pond.
As you will also be emptying the pool sadly you should evict the frogs, because in mating season, a frog can kill a fish (That’s not to say they will, but its not unusual) In fact as you have frogs and plants why not make another small “natural pond” and put them in this.

I would also suggest you read the following articles, and if you have any questions, ASK, but do take pictures as you start and as you go, and post them here.

Koi Ponds

UV and bio filter

I also ask what size is the filter and pump? (If you don’t know the size, make and model will do)

Many thanks for your expert advice 2010,

It may not seem it, I did actually read up on keeping Koi before I bought them and there seemed some ambiguity on how to keep them, some said a murky muddy pond (as previously mentioned with mud/clad thrown in) was their natural habitat, I placed stones over the base of the plants & they all seemed to do well. (No, I didn't know they took in oxygen at night) The other side advocated a clear pond so the fish could be kept lets say in artificial conditions with no plants & if I remember correctly some chemical additions. I must say I followed the former as is clear from my monologue. The 13 fish actually survived for over a year before dying over a few weeks (the last 3-5 died all in one week).
I respect and will follow your advice if no others advocate trying something different.
Something I forgot to mention: Before the fish started to die I introduced some duckweed from my water butts, this soon proliferated and almost covered the pond quickly, I use a net occasionally to try and keep it down somewhat, my analysis was that this may be the problem,that by covering the surface it assisted with shade for the fish but may have stopped the oxygen getting into the pond or/and may have been poisonous for the fish? I have also varieties of freshwater snails.

Thanks for the links I will read with interest.

Filter: Filtoclear 15000
Pump: Oase Aquamax 16000
Airiator: Superfish Koiflow 20

I'm obviously throwing every last thing at this problem before taking your very valued advice.

Hi EagleC & 2010, an update for you ....have just tested my water PH balance and its shows around 6.25...I've just read that a good healthy pond water Ph should be around 7.25 - 9.5? Would this level reflect the high ammonia levels that EagleC kindly suggested it might be? (Got tadpoles by the dozen and no more dead fish but still managed to man handle one fish today) Thanks

You really should empty and clean the pool and get that filter up and running before you do anything else. (Nice choice of filter and pump by the way)
As for adding duckweed most people spend their lives trying to get it out, not adding it. (As you found, it multiplies very fast)
I also get the impression you are not sure what to believe, ie who is right.
Plants V no plants etc.
I shall explain. A lot of Japanese koi farmers do indeed have “soil” in the water this is because the ponds really are natural, no fibreglass pools, no filtration etc, (Some places you can’t see the fish because of the amount of soil in the pond) But what is often never mentioned is the water to fish ratio.
In a “European koi pond” its not unusual to have very high stocking levels, koi produce a lot of waste (and I mean a lot) the “European” way to compensate is to have a lot of filtration, but as I mentioned Japanese koi farmers basically have big natural ponds with “few fish”
Or to put it another way, you live in a house, you can never leave the house, never. All food etc is brought to you, you live a happy life. Er, I “forgot” one thing, there is no toilet, you will be ok for a while but then what? It’s the same for your koi, without the filter…………

Please also read (again) the post about UV’s

One other thing, koi are a “very fussy” fish, that’s also why you should have nothing else in the pool, it eliminates any potential problems. If properly looked after koi will take food from your hand.

It does sound like the plants were a contributing factor. When plant matter breaks down this produces ammonia, adding to the bioload in your pond. The oxygen exchange is also stifled when the surface is covered with plants, plus as previously mentioned oxygen will be removed over night. I'd say this change in water chemistry and the fact you had no filtration running which (with a decent population of aerobic bacteria) would have coped with the increase in bioload from the plant matter. To a certain extent the mud would have acted as a natural filter but it's a poor filter in an artificial pond with a larger concentration of fish than you would get in the wild, this is why people tend to use an artificial filter. I can only really echo what has previously been said about the long term build up over the course of the year, coupled with this change in the nitrogen cycle has caused loss of oxygen and/or an increase in ammonia.