One of my fish just died of swimbladder disease.
How did this happen, and is it contagious?
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Swim bladder disease, or disorder, isn't necessarily any one thing, it's basically the result of some other condition adversely affecting the swimbladder of the fish. The swimbladder is an internal organ in fish which is used to increase or decrease buoyancy of the fish enabling it to maintain itself at a depth in the water without specific effort. Failure there can cause the fish to basically belly-up on the surface of the water.
Common causes include:
Parasites - intestinal parasites in the fish infecting the bladder. This could be potentially contagious in the sense that one fish infected by parasites is likely not unique in the water conditions. However, it's not fish-to-fish contagious.
Constipation which causes some bloat that applies pressure to the bladder that renders them unable to control it. Constipation isn't, of course, contagious, but the reason for the constipation should be determined to avoid causing it in others. Probably means a reduction in food amounts.
I would hazard #2 is most likely the cause. It would not surprise me that overfeeding would be an issue for most, it's relatively easy to do by mistake.
Swimbladder disease is a bit of a catch-all term that can be applied to assorted problems with the swim bladder.
You can tell when your fish has a swim bladder problem because it will struggle to keep its balance in the water, showing signs like falling to the bottom of the tank and failing to rise, floating near the top, or swimming on its side or with its tail higher than its head.
The cause of a swim bladder disorder is the swim bladder either getting swollen or compressed. Compressing or relaxing the swim bladder is what enables the fish to maintain and change its buoyancy in the water, so if the swim bladder is inflamed or squashed from other sources, naturally this will limit the fish's ability to do so.
It can be compressed from the swelling of other organs - stomach, intestines, or even liver or kidneys. Most common is swelling of the intestines due to constipation, or swollen stomach from eating too quickly or gulping air.
The swim bladder itself can also become swollen due to infection or parasites.
Determining whether it is contagious or not depends on the root cause of the swim bladder problem. If it is a single fish with constipation or swollen stomach from gulping air, it's unlikely to affect the other fish. However, if it is an environmental factor, chances are the other fish in the tank have also been exposed to it and you should look into remedying that.