Jumpy bugs

["Davorin Palijan" <davorin.palijan at os_tel.hr>]

Hello everybody,

I'm not very active on the list, but you'll be glad to hear
it's "first thing in the morning" thing for me to  read it.
Thank you people, it is invaluable source of knowledge.

Question: Few months ago I noticed, during maintenance,
that some kind of small (very small - like a dot in the 12pt size text)
bugs appearead on the water surface in my planted tank. 
They are grey in color, jump around and sit on the 
water surface and glass. It is very hard to look at them from 
close range - they just "disappear".
The problem is they multiplied, and although I cannot
see any ill effect of them living in my aquarium, I would
like to get rid of them before they multiply too much.
Anybody knows what these are?

Thank you.

Davorin P.