death of corys can't understand

Shekoi

Trying to start up again
i've had my 7 cory juilis for nearly two years

two died a week or so ago, which i put down to being stressed by my bristlenose pleco, who then moved into different tank, and all was fine.

then yesterday i noticed one of the corys swimming upside down unable to correct it's self, when i got back from work it was dead. today i've noticed a second cory doing the same, he keeps sticking himself under leaves to stop himself from floating upside down.

the other corys seem fine eating and acting like normal?
the water readings are fine, i've down nothing different in terms of water changes or food.

What is the problem?

Please help Shekoi.
 
Although some corys can live to be 10 years old, its probably more common to see a 2-5 year lifespan.
 
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