Ahhh... that looks a lot different than the first photo- much clearer.
So they're not actually below the water line- they're ABOVE the water line? That explains why fish arn't eating them... it's probably more of a general terrestrial plant pest than an aquatic one- probably just a coincidence it's in your aquarium.
Definately looks like an insect of some kind. If I'm seeing wings not shadows- that looks like a fly of some kind.
You might get more help from a gardening forum- they're probably more familiar with them.
I got the same bugs, i believe they are aphids and mites.The only way to get rid of them is by manual removal.Funny, the only thing thats eats them are lady bugs and i have a couple that have been sticking around