That is the reason that I tried to clear my small pond as I knew it was very stagnant and I wanted to clean it before the frogs started spawning. I only just did it in time. They were already mating as I emptied it but they were happy enough to ignore me as the water disappeared and clean went in. The only other thing that I would have done if I had been too late would have been to introduce more clean water hoping that as it overflowed the most stagnant smells etc would float away.
It is lovely to see you back posting Damp. You are always such an inspiration to me how you look after the environment and are concerned for all wildlife. Keep it up.