In looking under the Tropical Fish diseases, you will find notes that swim bladder can be caused by many things, and it may just be pertinent to the fish themselves. Or it may be more like a symptom of other ailments such as bacterial or fungus infection. I can't tell you from which, but you might consider watching fish closely to see if any other signs show up, such as ICH, fungus, or situations like fin rot, bruising from very aggressive fish, popeye, etc. If any are seen, then you should treat the whole tank. Otherwise consider the possibility that only the Corys had it, and the rest are fine. It could be that the Corys had it already, and had it longer than you think, but it just showed up when it got severe enough.
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