What Could Be Wrong?

[ellena]

Last week one of my three bettas in a divided tank went into hiding. He comes out to be fed and has a bit of a flare, looks fine, then hides again. I noticed the tiniest bit of discolouration on his tail, so I did a myxazin bath. His tail has started to disappear, so I've kept the baths up half hour a day.
Could be chewing it, but it was discoloured first and there seem to be bits where I don't think he could reach.
So finrot? Do they usually change behaviour with that?

 

[Wildbetta]

Can you possibly get a picture of him?  That would help in determining if he is suffering from fin rot or from biting.  Have you had any temperature fluctuations lately?  What is the temperature that you currently keep the tank at? 
 
I suggest you turn the light off and shine a flashlight on him (and the other two as well) from an angle and see if you see anything on him that looks like "dust".  This could look grey, whitish, or even goldish in color.