I recently bought this guppy at a local fish store about 3 days ago, and during the first two days he had been swimming around the tank no problem, only today he has been staying at the surface of the water and has not come down once. I do not think this is an oxygen issue, as I have several other fish swimming around just fine. He is moving around, but only skimming the top. My ammonia and nitrites and nitrates all tested 0, I could use some help identifying what the problem with him is. The tank is a 20gl.