Bettas losing their tails!!

Question
I have 17 bettas, all live in bowls ranging in
size from about 1 quart to 1gal. I have noticed
that a few have just all of a sudden 'lost' their
tails! What I mean is that they have lost about 1
inch of their tails and usually no more. No other
fins are lost in fact its ONLY their tail that seems to just 'dissolve' away. What could this be?
Twice a week I change all the bettas water doing
100% water changes, adding de-chlorinated room temperature water. I feed them once a day with
Hikari brand betta food. I do nothing to the water
except water conditioner. I don't understand whats
wrong. I keep eight other fish tanks and have no
problem with the occupants fins or anything.
Its strange how just a few will get this weird
'tail rot' why would it be just the tail?
I use de-chlorinater that also removes ammonia
and I am careful about feeding. It seems strange
that when I do all I can to take the best care of
them it makes them act strange.
Is it something to do with my tapwater?
Would ageing the water be worth a try?
My bettas fins don't seem to grow back much.
Their tails aren't ragged just shorter than usual.
I'm worried my beautiful pure white bettas
are going to get this problem, please if their are
any suggestions you may have please let me know.

Thank you for your patience to read this long letter
and as always, thank you for your help.
sincerely,
Susan*

Answer
Hi Susan;

There are several possibilities;
They may be biting their own tails off thinking there is another betta in with them.
There are plastic plants in the tanks tearing them.
They have weak tails.

Do you see the pieces of them in the bottom of their tanks? If not, they are probably eating them. If you do see pieces, they are being torn off somehow.

Followups welcome.

At Your Service;
Chris Robbins