Greenfly infestation

[octofasciatus]

I seem to have an infestation of what i can only describe as greenfly. Advise please on what to use to get rid of the beasties. I dont know how they have come to accumulate in the tank, all my plants are below water level. Every morning i wipe the excess greenfly from the inside of the lid where they seem to accumulate. Dont really want to use something that will harm the fish.
**Please no windups - i.e it's really the Malaysian parasitic brain eating vampire fly - I will not find that funny.
Ta

 

[Nabbig2]

Pictures will help.

 

[santoury]

They are there because something attracted them - I don't know the lifecycle of these flies, but most flies are attracted to carrion, dirty/stagnant water, rotting matter. If there's any of this in your tanks... obviously clean it out.