hello. I have been breeding corydoras the past few years, owned hundreds of corydoras, and have never seen this before. I have a free week old fish who has been swimming at the very top of the tank, upside down, or poking at the water's surface. It won't do this all the time, but only every 10-20 mins. There are other corydoras and BN plecos in the tank as well, which do not show any symptoms. Tank is 60 gallon. Ammonia and nitrite are testing zero, with nitrate testing very low. Parent fishes in the tank together are showing breeding behaviour at the moment, so I am doubtful this is water quality issues. Anyone seen this type of thing before?