A bent spine IS a deformity. To keep DT a strong line you absolutely positively MUST ensure only the BEST fry survive. Now, if there are mitten tails they will be given to people on here as only pets, but deformities no. Some person will probably say "hey! A doubletail! I should breed him." Not only disappoints the owner when all the fry die, but when the betta dies, and would not benefit the DT line at all. It would make it worse. This fry has a short body, hence the bent spine. Whose to say he/she won't suddenly die?
I was soft with Janey's last spawn. I should have culled way over half. And only the strongest really did survive. And I've learned not to be soft. I can't keep every deformed fry.... Live threatening or not. DT by itself is a deformity, and letting a deformed fry go is just asking for trouble.