Fresh or salt water? If it's salt water, I've found that my fish go near the top when the salinity is too high, and stay near the heater when the ph is off. When I came back from school I noticed some of my fish staying near the top so I figured that my dad had let the salinity get too high, so I check it, and it is .39 on the specific gravity meter (think dead sea) so I drain out about 5-10 gallons and pump fresh water into the corner of the tank, well long story short I did it too fast, the one fish went down to the bottom of the tank then swam as fast as he cold towards the top, made it halfway out of the water and literally died in the air, and just sunk when he came down. Craziest thing I've ever seen from a fish heh. Oh if its freshwater, yeah the fish is on his way out, if its just a gold fish flush him, don't want him around healthy fish, if its something a little more expensive put together a quarantine tank.
