Neon fish attacking the Betta male

[Christine100]

We had a beautiful Betta male and a big aquarium (200 liters) with natural healthy plants, plenty of places to hide, great filtering system, the water is clean and warm enough, great lighting, not many fish (15 small neons,7 tetras, 2 cleaners, 2 small Angelfish and a Betta male). All was fine for weeks until one neon started attacking and nipping the tale of our Betta and all other neons started doing the same. We removed the most aggressive neons few times out of the aquarium as a punishment and when we took them back, they were calm for a while and left him alone, but would start attacking him again after a few days. We bought a separate tank for the Betta with a filter, a heater, light, natural plants and a rock to hide. However, he was already under a lot of stress, started losing scales, we treated him, but then he died yesterday.
We want to buy a new Betta male because I love them, but what to do so the neons don't attack him? Is it even possible to keep them in the same fish tank?

 

[Brookster123]

This is why Bettas are best kept solitary. If they're not beating on the rest..the rest are beating on them.

Sent from my SAMSUNG-SM-G891A using Aquarium Advice mobile app

 

[Christine100]

This is why Bettas are best kept solitary. If they're not beating on the rest..the rest are beating on them.

Sent from my SAMSUNG-SM-G891A using Aquarium Advice mobile app

The problem is that the Betta attacked them first and established domination, but that quickly changed for no reason. Is it because the Betta is slow and neon just love to nibble others? :/