My 12 Inch Silver Aro biting its own tail...

any bros here know y? im worried coz its tail look aggrevated now

my opinions...
1. its itchy or annoyed at it?
2. it outgrew my tank already thus each time making a turn seeing its own tail, and starts to chase it?
3. internal bacteria? coz i took a very close look at it, it has no external parasite like ich etc etc...
4. BAD WATER PARAMATER (my last bet tho coz all other fish in tank, 5barbs,1feifeng,1tri-color shark and are okay...

hmmm... help any bros here?

lol u guys!
my tank is 55gallons 4x1x1.5
poor tail... he stopped biting it as for now.
will it grow back to normal?
do i have to do anything?

thanks man!