The fish cannot hold position — floating at the surface, sinking, listing to one side or tumbling.
Short answer
'Swim bladder disease' is a description of a symptom, not a diagnosis. Buoyancy problems arise from digestion, from physical compression, from infection and from selective breeding for body shape, and the responses differ.
Ordered by urgency, then by how commonly each one turns out to be the answer. The discriminator is the observation that separates this cause from the others — use it before acting.
How to tell: Multiple fish affected suddenly, often after a water change with mismatched temperature or untreated water.
How to tell: Persistent, not related to feeding, often with other signs — bloating, raised scales, lethargy, loss of appetite.
How to tell: A fancy goldfish or balloon-bodied strain. The problem appears after feeding and improves between meals. The fish is otherwise alert.
How to tell: Herbivorous or omnivorous fish fed a protein-heavy diet, with reduced or stringy faeces.
Checked before anything else, because a keeper who learns their swollen fish is carrying eggs and their spotted fish is a breeding male has been spared two unnecessary medications.
How to tell it apart: Many catfish, loaches and gouramis rest on the substrate or propped against decor at an angle, and some rest on their side. If the fish swims normally when it chooses to move, it is resting rather than struggling.
Each entry states the observation that separates it from the others, and links to the full record. Work through them before deciding what you are treating.
How to tell it apart: The full account of why loss of buoyancy is an appearance with mechanical, structural and pathological causes rather than one disease — and why which of the three applies decides everything you do next.
How to tell it apart: Disorientation affecting several fish at once, with gasping and reddened gills. Test the water before assuming this is a buoyancy problem at all.
How to tell it apart: Sudden onset across every fish immediately after a water change or a heater event, with rolling and loss of coordination rather than a stable floating or sinking posture.
AquariumHQ describes what is commonly reported and what husbandry correction is usually indicated. It does not diagnose and is not a substitute for an aquatic veterinarian. Many treatments are given to healthy fish suffering from a water-quality problem, which makes the outcome worse rather than better.