This seemed to have recently started and im not quite sure what the cause is. He seems to be still fairly active but he's swimming by like wiggling and keeps flipping upside down. His fins are all intact so I know that's not the issue. I'm not sure whats going on or how serious this is so any help would be appreciated. He is in a planted tank 20 gallon with 4 other danios and a female betta. The other fish seem fine. It just looks like he is affected.