.... Cardinal tetra can live to around 4 years. Think there might be some problems with your tetras for them to die after only 8 months.
1)Maybe the exact age of the fishes you have is around there, 4 years old from fries to full adult....
2) overeating ????
3) Prolong exposure to certain addictive/metal/chemicals/chlorine ? 8 months worth of poisoning.... newer fish might not exhibit until lethal dosage is accumulated...
4) Bad luck ? PH crash ? Ammonia spike ? Internal blockage(E.g like heart attack, too much fat/food ) ?
More investigation into tank params might shine more light into it. Top priority is always the water quality, then followed by care of the aquarist. ( proper tank maintainence, avoid overfeeding , etc)
My thoughts
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