Check everywhere in the aquarium, every single nook and cranny including the filter, even if you think there's a spot that there is no way it could possibly be. Then do the same thing in the immediate vicinity of the aquarium. Don't forget to check your nets and buckets either. After that, start looking around the rest of whatever room you have it in. If you don't see it, you either didn't look hard enough or you washed it away with the water change.