Nitrate poisoned again???

Question
My Cory catfish is floating upside down! The platies are fine but the Cory isn't! When he swims he does it upside down. I just noticed it today.
Is it Nitrate poisoning?
And
What should I do?

Answer
Yes.  It is nitrite poisoning, not nitrate...ammonia/nitrite most likely, but could be any of the three, or a combination.

Change all the water and clean the gravel.  Wash out the filter.

He will be fine in about a week if his tank is cleaned out.