He's fin biting unfortunately. It's not fin rot at the moment though it can progress to fin rot if your water parameters not good. Just make sure you feed him well and do your weekly water changes to keep his water good.
You can feed him as much as he will eat in one sitting, generally my fish will eat around 10 pellets OR 4-6 bloodworms (frozen but they are thawed) depending on their age and size. But you can tell when they're done if their tummy is nice and round, not super large like they are bloated, but just a gentle roundness.
fin biting? oh... but I never saw him biting his tail.... I'll try to feed him more live food... he has a lot of plants... maybe he hurt himself (my mom suspects Eleocharis acicularis, because it's like grass. You can cut yourself with grass^^...)
thank you for your quick answer
greetings from germany
I just want to give an update.
My betta's tail is a lot better! It's still visible where he bit his tail but at least his tail is whole again^^
Live food also helped a lot. Searching for food in his aquarium is his hobby nowadays.
Thanks a lot, guys
Greetings from Benjamin Blue and Chiara