Take it out of the tank and examine it. I missed the wings the first time, but thought it looked more like an insect than anything.
IF it's an arthropod, I would think it might be a bad one (ie parasitic isopod) but it doesn't quite look like any that I've seen (in photos - haven't encountered one for real - knock on wood).
I'm thinking it's a drowned insect.
Jenn