Pond volume is 5000 litres and approx 20 koi of which the majority are 30cm. We had a power failure due to cable theft which lasted 48 hours. I squirted water into the pond with the hosepipe to try and oxygenate the water which to some degree worked. After 24 hours at day break two koi had jumped out the pond during the night and died and the remaining fish were really gasping for air. I followed the same procedure with the hosepipe. When the power was restored things were starting to return to normal, I thought. Two more fish died after that after appearing to be fully recovered. The ammonia, nitrate and pH tested normal. I washed the filters out and did a 30% water change. I have in total lost 6 fish. All the fish lost were the 30cm ones and not the smaller fish (15cm). Can anyone explain to me why the fish still died after appearing normal and feeding normal. After the water change there has been no further loses yet.

Hi Daan,
I have no idea why the 2 died, could be that the stress was too much for their system and they just gave up.
You had too many koi in the pond, I would suggest not more than 10 at 30cm each, just in case.
Sorry for your loss.
Regards,

We lost many fish with all the blackouts last year, we bought a generator! For the fish lol