I've been on this forum a longtime and have never entered one of these competitions before so thought I'd give it a try! This is my crazy cat Jess, he will be 7 years old in August so will soon be a senior :) the day this photo was taken he had just come back inside covered in what I believe to...
I've also had this happen to me, I tried with them twice as they are so beautiful but both times I eventually lost them all displaying same the symptoms.
I also got fry that I managed to grow into adults but they too succumbed to the same thing. I won't get them again for this reason, although...
I have a rabbit that does a good job of fertilising my garden with his poo and pee until he ate all my tomato and pepper plants so now I have nothing to fertilise 😭
It could be a flow issue as well, turn down the filter flow or turn it off for a bit just to see if this has an effect on the betta, they really struggle with any flow, a photo of the tank and fish will help too.
I see you have mentioned the flow is gentle but his behaviour is saying otherwise...