Dying fish

[Chellie]

Help! We have a 9000l pond with 2 large koi approx 35cm and had approx 28 Shubunkins varying from 3cm to 20 cm.
In the last week we have had 20 fish die. We have checked water all ok and had it checked by fish shop, done a partial water change, cleaned filter, pump etc (which weren't too bad) and added air stones. It has a fountain, waterfall and pipe circulating the water too. We have many plants and a large parasol creating shade from afternoon sun.
Some of the fish have had swollen bellies so we thought egg bound/aggressive male but not sure it's that due to losing small fish of approx 3cm and not all are swollen. Have treated with Fungus/bacterial treatment as advised by fish shop but still dying. We are devastated and really need some advise. Many thanks in anticipation Chellie

 

[Mmathis]

Hello and welcome! So sorry you are having problems!

You said your water tested “OK” per fish shop. Do you happen to know WHAT they tested for, and WHAT the actual results were? The numbers of the tests can sometimes tell the story (ammonia, nitrites, nitrates, pH, KH)

Has anything recently changed with your pond,

• such as added any new fish,
• added any chemicals (for any reason)
• big changes is your weather,
• pesticide/fertilizer use in the area?
• Oh, and how long has the pond been up and running?

Also, some pictures of your pond as well as some of the fish might be helpful.

 

[j.w]

@Chellie
I do not know what is happening and hope someone will be able to help you soon.

 

[poconojoe]

That's terrible.

It sounds like something sudden. That is, if all the deaths occurred in close proximity, time wise.
If so, I'm thinking something toxic got into the pond. Perhaps from ground run-off, as @Mmathis eluded.

It's also possible your pond has reached a tipping point where the fish have grown and also multiplied rendering your filtration inadequate. Perhaps a huge increase of ammonia?

Without knowing the exact readings of your water test, it's hard for us to guess.
I suggest you get a pond master test kit. API is a popular brand. They used to be about $20, but now they are around $30-$35. You want the type with liquid reagents, then post up the results. Dont use the type with the test strips, they tend to be inaccurate.

I hope you didn't add any chemicals such as algecide for example. There are way too many post here where so-called fish safe algecides were used and killed off people's fish.

Sudden changes in Ph can also harm fish.
For example, If your Ph was off and you added something that altered it too quickly.