fruit flies in my fish tank?

[stir-fry]

while doing a water change today, i noticed some little critters on the inside of my tank, really close to the top of it.
They resemble fruit flies..

how common is this?? I don't think i've ever seen it before.

[Carol_Roberts]

Flies are not a good sign. Time for a major cleaning. Clean the gravel, the insides of the tank, rinse out the filter box and rinse the media in dechlorinated water. Feed less, clean more, flies gone ;D