What are these small black insects living in my pond and eating the algae ? Scotland question.?

these little black insects are living on top of the pond theres a group of them eating the algie that and they can swim down to the bottom of the pond they kind of wriggle and dart about and go upside down occasionally when they move and it looks like they are white underneath, these insects are really small about 1 centimetre in length and very thin about the thickness of a thick pin for sewing, im just wondering what these are as i only cleaned the pond out, re sealed it and put new water in 3 weeks ago and there is algie growing inside it, white pond slugs and allsorts in it alread ytho i have never noticed these little black things in the pond before, then again iv never really noticed having algae in the pond either. I live in Scotland if thats any help.

• Answerer
  Do they look anything like these?
• http://www.nationalinsectweek.co.uk/caug…