Question
I have a dam approx 200 000 l water. all my koi fish died in a matter of 1-2 days (80  > than 40 cm large fish) I believe it could be because I sprayed ROUNDUP/GLYCOPHOSPHATE on the garden close to the dam, and suspect some of the poison was wash into the dam when there was substantial rain two days after the poison was sprayed.The poison was 1.5 l to 100 l water dilution, and only about 100 ml was used around the dam. The fish died 5 days after the spray, two days after the rain. My question 1) do you agree it was the poison or shall I look for something else ? 2) if it was the poison, how long before I could fish in the dam again ?
Thank you for your time and effort. Wolf du Plessis.

Answer
That is about a 1:500,000 l ratio poison to water.  That product could definitely do some damage, but I don't see it being fatal in that dosage.  Glycophosphate is one of the most acidic substances most humans will ever handle, at about 2.0 pH.  It is possible that it caused the pH to drop considerably.  There also may have been one fish carrying a bacterial infection, and when the conditions caused their immune systems to fail, the disease spread.  Look to see if the fish are especially hard and lack their slimy exterior.  So, I don't expect direct poisoning, but it may have thrown the water chemistry off enough to cause the problem.