I went for a walk locally and on route noticed a garden pond sited in a lovely setting, but unfortunately it had become neglected. The water had become mirky dark colour. I didn’t get close enough to detect whether it was foul smelling too.
The greatest joy of a water garden is the water itself. Pond water which has gone a black colour often with an oily film around the edges too, is usually the result of decaying organic matter such as decaying leaves. This is quite possible at this time of the year if leaves and other plant matter aren’t skimmed off the water surface before sinking to the bottom of the pond.
A thorough clean out of this pond would be the only answer!