Question
The pond water in the garden turned black overnight last week and killed one of our koi, the wierd thing is, the same thing happened 10 miles away at a friends house, we both had to empty out the ponds several times before the water was clear again, luckily we only lost one fish, but our friend lost half of his koi. Nothing had changed in the water and since then its been fine. Our friend thought maybe a plane had dumped some fuel, what do you think caused it? weve had it 4 years and had no problems at all in that time. Thanks

Answer
Dear Rosie,

It is hard to tell from A distance, however I have experienced something likewise in Belgium.
The reason was A large change of weather and more exact it was A normal day going over in real thunder and lightning.
This caused A reduction of large amounts of oxygen in the pond that killed fish and also some Algae.
These algae turned darkbrown.
Was there A weatherchange in the last days?
Was there visually something to see at the fish like hanging, flashing, jumping?
What are the water vallues like pH, Ammonia?

Kind regards,
Raf