What's that in the pond?

We have had a pond in our garden for a couple of months now and I've been deciding if I should put fish in it or not. This afternoon some friends came for tea and noticed movement in the pond. I think this is mayfly nymph and there was also some mosquito lavae (not so exciting). I had great fun trying to photograph them - nearly falling in as I tried to get closer!
Any top tips on ponds gratefully received!