So, my new fish that I got two days ago isn't doing so well. Yesterday he was great, really active and seeming to enjoy his new tank. Today I got back from going on a sudden overnight trip, and things have gotten bad. He's been hanging around at the top, and I thought he was just being odd, and then I noticed he's not staying on the top because he wants to, it's because he can't go under. He sits for a few seconds, tries to submerge, and then floats back up. I know this isn't normal, so is there anything it could be, and what I can do to help, if anything?