Anisus leucostoma is planorbid freshwater snail with Palearctic distribution. It occurs in standing water such as periodic marshes.
Its shell is flattened and it has dimensions of shell about 6.5 mm × 1.4 mm. Therefore it is not easy to get its sharp photo with such dimensions and moreover in water. Addtionally its brown shell can be covered with some coating, that gives to the shell very dark appearance.
Anisus leucostoma crawling below the water surface in natural light conditions. There is also visible for example mantle edge on the lip of the aperture:
Comparison with larva of Culex:
Anisus leucostoma extended from its shell:
There is periphyton attached on the shell:
Under bright artificial lighting and with white balance there is visible, that the periphyton consist of green algae:
gastropods 
I have taken a look at one of the photo captioned "There is periphyton attached on the shell". I think, that there are structures called as "hairs" on the keel. Hairs are known to be on the keel of some species of Planorbidae, but I have not seen hairs on this species yet.