What do you mean by a "controllable" swim bladder? They're relatively primitive fish closely related to arowanas. So they're members of a group called the physostomous fish that control the amount of air in their swim bladders by either gulping air (if they need to inflate the swim bladder) or burping (if they need to remove air from the swim bladder). This contrasts with the more advanced physoclistous (such as cichlids) that are able to add or remove gas from their swim bladders using a modified system of blood vessels around the swim bladder. This is why cichlid fry don't need to rush to the surface to inflate their swim bladders after their have hatched, whereas baby minnows and catfish do, and will not develop properly if the water is so deep they can't make this journey.
Like most fish, they're neutrally bouyant -- i.e., they neither float nor sink -- they just happen to prefer to stay close to the surface of the water because that's where their prey is. Wild fish feed almost entirely on aquatic insect larvae and aerial insects that have fallen onto the water. They jump up out of the water at times, certainly to escape from aquatic predators, and possibly also to catch insects flying above the water's surface.
Cheers, Neale
Do these fish have a controllable swim bladder in the normal sense? Reason I ask is because they never seem to leave the surface.