Fish dying :o(

[kimi730f1]

Hi all,
I am new to forums but I am desperately seeking help. I have a pond roughly 10,000 liters last night my beloved sturgeon died and today my other sturgeon has died along with 3 koi and my other fishre not looking very healthy - most are just lay motionless. I am sorry but I really do not know what information will help but I will start with the filter, pump etc. The pond has a Biotec 10.1 Filter with Bitron 24w ultra-violet unit and an Aquamax Eco 8500 Pump. I also run a fountain from a Blagdon 1500 pump during most days. Last year I had hundreds of pounds worth of koi die in exactly the same way they are dying now. I have checked the water quality which is fine, I have two big air stones and two waterfalls. (I have attached pics). View attachment 2917

View attachment 2918
The fish have no spots blemishes etc they look perfect even the dead ones. I do have bark mulch in the garden although we didn't last year when apart from 3 goldfish my entire pond stock died within 3 days the same way things are happening today. Just over a week ago I added cloverleaf Blanket Answer, although this was not added last year. I would be grateful if anyone can help give me any ideas to try and find an answer to my pond problem and help my fish. I can only apologize if I come across as not knowing alot but I do however love my fish.
Thank you very much for your time.

 

[koikeepr]

Let's start at the beginning to figure this out.

What are you water parameters? Get a test kit and give us the figures for ammonia, nitrate, nitrites, Ph, etc. This will tell us a lot.

Have you been doing weekly water changes of 20%?

 

[kimi730f1]

Thank you, I have cleaned the bottom of the pond and done a part water change. I went out today and bought a new water testing kit as my other was around 2 years old so I was unsure how accurate it would be, I have tested the water today and the levels are as follows:
PH - 7.5
Ammonia - 0
Nitrite - 0.1
Nitrate - between 5 and 10
Thank you