Thanks Nossie and Andrea,
this is the 20l tank we got from our neighbours, we first wanted it for a Quarantine tank, but when this happened to Barbie, we decided to have it running for him.
Those tiny Rasboras are cute, so the panda corys, they don't grow big either.
Yes I did treat with BetaFix. I think this was the thing to get him over.
Aparently the red edges will grow back, but it will take weeks. We spoil our boy with Betta pellets, Bloodworms, Blackworms, Betta flakes.