I agree with elise. I doubt it is finrot, probably just a tear from the plant. It *could* develop into finrot, but given your routine with his water and salt, I doubt that. Keep up the good work!!
Oh, you could also add a little extra stress coat, or similar product. It helps form their slime coats which can help them heal. <img src="/images/graemlins/smile.gif" alt="" />