I don't believe mosquito larvae have pairs of legs on the first 3 segments. That excludes the mosquito.
A water beetle is a good guess, perhaps the http://www.bgsd.k12.wa.us/hml/jr_cam/macros/efl_spr06/efl_spr06.html" . (scroll down that page)
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Caddisfly[/URL] larvae also have 3 pairs of legs on 1st three segments. (scroll down to 'Order Trichoptera'). They like fast moving water from which they filter plankton to eat.
Larvae generally go through several molts (in star) stages, getting larger with each successive stage, so the 1/8in long larva, may be one of the intermediary stages. If you want see the adult; it is finding food where you found it, just let it grow. You probably want to put fine screening material around the tank, such that the adult won't be able to fly away, until after you've studied it.