Betta constanly tears fin, lost to why?

I have a 15 gallon with a betta, 10 neon tetras and 6 platys (all except one are juveniles, I will rehome most/all in the near future).
every week or two my betta has another portion of his fin torn, sometime its pieces missing but sometimes nothing is missing and it just looks separated and shredded.
he loves sleeping on the filter and to get there he passes through a narrow passage between the filter and the glass. i removed any decoration I deemed as problematic but there might be more I'm missing. haven't noticed any aggression or fin nipping. what do you think? fin nipping? the filter? decorations? i don't want to be forced to separate him but I will do so if I cant solve it, willing to remove fin nippers to other tanks
p.s i have 2 cpo but they are terrified of him, they sometimes show their claws when he passes but I don't think they attack him

 

Most likely i would bet on nipping either from hinself or the neons or the filter, depending what type it is.

 

that doesn't narrow it down at all......
one time he was completely missing a single tiny piece of his caudal fin
then after 2 weeks he was missing most of his pelvic fin
then today (a week and a half later) his caudal fin didn't lose any fin, but it looks separated and shredded (but besides the change in look its intact)
p.s he seems pretty active (compared to a betta) eats etc. so I guess it's not self inflicted

 

so I guess it's not self inflicted

Wrongo on that one. I am battling fin biting myself right now. He eats every day and is extremely active for a male betta. I have lots of rocks and tunnels for him to play in and he still bites chunks out daily.

 

ok so I added a few pictures to clarify (I guess it will not help but still better to try than not)
another question, is it in any way life-threatening (even in the long run) or should I be calm, keep good water parameters and try to solve it before separating him?

 

Separating him would definitely tell you if he was doing it or someone or something else was. Just keep calm and keep his water pristine. It would only be life threatening if he couldnt get to the top for air or it gets infected.

 

noticed one of the tetras with a partly missing dorsal fin
so does that mean one of the tetras is to blame for the betta fin loss?
would this fin grow back?
what should i do with the betta?