Lots of tiny black like bug things in my pond why?

My goldfish have been there for 2 days (they are still not moving from the bottom all 10 of them) the pond has had water in it for 4 weeks before puttin fish in and theres been no black bug things but this mornign i went out and theres hundreds of theses black things what are they? are they harmful to ym fish? also the fishfood has gone and i don't know if the fish ate it or the weird bugs please help