I just noticed a small black outline to my red bettas tail. I noticed this a week ago and thought the light was just playing tricks with me. I can see it better now that I have moved his tank. I am so worried! I dont know if its just his tail-coloring because the black outlining is very small. All of his other fins are fine, its just the lower part of his tail fin that has the tiny black outline. His tail hasnt gotten any smaller or anything. Help! 
