Maybe the O-ring is not seated right or torn that seals the cover? I know there's always a risk when you have a big fragile vessel of water in your
My 4 emerald corys stay close enough to touch each other almost constantly. I guess they're not true corydoras though.
Never heard of them being like that. I know they are territorial and need to be kept either in groups or solo also gotta have a higher female then male
I spot clean where possible. Most important cleaning is in terms of water change. I replace 50-75% weekly.
Correction: Nitrate is 20.