Here is a single line, and it does a lot with very little. The picture is simple enough: gills, the feathery slits on a fish, catching the light and flashing. But the verb "tilted" shifts things. Something is leaning, and it leans "in their favor," which is the language of rigged games, fixed contests, and lucky breaks. So the shimmer of the gills stops being only a pretty detail and starts to feel like an advantage.
Who "they" are is never said, and that gap is part of the point. The poem doesn't tell us what the speaker feels, and the tone is cool and dry, almost like a statement pulled from an argument (the title, "Thesis 9," suggests a numbered claim, though the poem gives no proof). The mood is strange and slightly playful. Natural beauty and a sense of unfairness sit side by side, and the reader is left to decide whether the glitter is a gift, a lure, or a trick.