I would def check the filter. Also, you might want to check to see if any rocks you might have in the tank have fallen over or just shifted. A sleeping fish can be surprised when its hiding spot falls upon it. If not, then sometimes they're just stuck in the strangest of places. Also, many times a rotting fish can look very similar to a piece of gravel, so it could be easily over looked (I've experience this myself to discover a couple days later that there it died and then the body just settled with the gravel and blended right in). Good luck with the search.