Thank you all for your answers.
My question has many goals, first one is to start a good discussion on this forum with experienced fish keepers.
I also like to learn new stuff and know more about the hobby and fish, therefore, I also ask controversial questions.
I don't mind badmouthing so keep everything comming
To give a better idea of why I would do something like that is that I have a very very beautiful Betta that got his tail torn so much from his past community experience that I had to move him in another tank all by himself. Now that his tail grew back, it actually looks really weird, rays are split into two in some places where it was initially torn. I want to cut it back to give it a chance to grow back nice and symmetrical.
Some people are making all types of comparaison with cutting a betta's tail and other animals. I'm asking because I am very concerned about hurting an animal.
I am strongly against cutting cat's claws and other attrocities of the sort, but I will trim their nails as I know this doesn't hurt them.
I also trim my dog's nails and it doesn't hurt him as well.
I'd really like to know if someone done it in the past and show some evidence that it hurt or not..
Thank you!