I've had two small goldfish in my 3 galloon eclipse system for over a month. One of the goldfish is perfectly fine, but for the past couple of weeks the other seems healthy but has a buoyancy issue where he can't help but float to the surface of the water. When I approach the tank, he swims around as if nothing's wrong, and eats normally too. When left alone, though, he just lays on his side at the surface, moving around from time to time. His breathing is at a normal rate. You can see him struggling to stay down in the depths of the tank at times, but he is unable too. What do you think the problem is?