if it's not a gibbis, and not a skinny rhom, the shape looks like one of those rhoms in the youtube vid where which showed wild rhoms from a tv show with an especially angled forhead and red eyes. never seen any other rhom pics online like them.
i bought a serra as a present for a buddy that I thought was a gibbus, exact same shape, not long enough to be an elong that resembles this fish. that fish had clear fins and eyes though, and red gill plate. He wanted to speed up it's growth and threw it in the pond, since then we haven't been able to find it.