I don't know the reason why he died, but I don't think the plastic plant made such a large hole in his tummy. I mean, maybe that's what happened, but I wouldn't think so. I'd guess another fish did that to the poor cory after he died. You only have (well, "had") 2 cories in the 20 gallon tank? No other fish?
I know you said your parameters are good, and I believe you because you said you perform 50% water changes every 5th day. So they've gotta be excellent. But would you mind posting your
ammonia,
nitrite, and
nitrate readings anyway?
I also have a question: If you only have 2 cories in a 20 gallon tank, why do you need such large water changes so often?
I'm sorry about your poor fish
