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Fish are poikilotherms and adjust to the ambient temperatures with ease unlike mammals who regulate their internal temperatures irrespective of their sorroundings. With this in mind, your fish being tropical would not really perform well in drastically colder environments. Check your heating connection to see whether it was a case of electrocution due to a leaking electric current. It could also be that overheating occurred that is, temperature shock if you increased the heat drastically over a short period of time. Another plausible cause of these deaths could be oxygen depletion. Heating water drives oxygen out and this could cause lethargy and asphyxiation.
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