What are stress lines?
I'm gonna take it as something that appear because the fish is stressed out. Then it probably due to the fact that I was on vacation for a week and neglected him ....:thumbsdown:
But he's active and eating , and seem very happy so he should be coming back soon,.... i hope
bettas get these horizontal lines when they are stressed and the area losses a bit of color. I wouldnt worry too much I bet he will be fine after some attention and a water changes!