There was a creek/sewer drain system by the house I grew up in. In summer, I would go down with my little brother and try to catch tadpoles. You could find them in the still spots, nestled in the muck, quiet and shiny. Sometimes you would have to step in the water and you couldn't see the bottom. My watersocks would come back grey with fine mud and strings of moss. Warm creek water, handfuls of fat strange tadpoles. Some with half a leg, or all legs and a useless tail. One summer, on the day that school got out, there were more baby frogs than there had ever been. You couldn't dodge them on your rollerblades. Hopping out of the creek and infesting the park, the sandbox, the drinking fountain. They had made the victorious mutation from water-breather to air-breather- but for what? To spend 4 August days as the dominant species of our neighborhood.
That's the stuff I was thinking of for this one. I wanted it uneasy and familiar.