balance

Hello! So I've been in the hobby for just over a year, made all the mistakes at the start and learned a LOT. I now have a nice healthy tank of fish, I wanted real plants and have planted the tank got it balanced not too badly(or it wasn't balanced in the first place and it was just luck) but...

Request Help   One of my two false bandit corys (corydoras melini) can't keep balance in the water, and frequently ends up losing balance and ending up on his side or upside down. He can right himself, and swim about normally, until he stops, at which point he has the issue. Tank size: 30cm x...