Too much fish food!
Why is it my grandson insists on grabbing huge handfuls of fish food, and chucking it into my pond, the little blighter. I've told him not to do it before. I know he enjoys feeding the fish, but I don't think he takes on the fact they aren't as hungry as he can be :o)
Actually I think he must have realised his mistake, because I later discovered he had used the fish net to remove some of the surplus and had tried to hide it in the bin. But there was still loads of pellets floating around on the surface.
Ah well, I'll just have to remind him again!