Just for grins, check your pond for macro-invertebrates or insect larva - particularly near the area where the tiles funnel water into the pond.
I'm thinking IF there's a contaminant (carbamate or organophosphate insecticides, at fish-lethal levels), the macros and insect larva would likely be absent. But, if live specimens are present, the probability of an insecticide contaminant would be slim.
This is simply a thought, and not based on prior experience.
