Hi guys. I have noticed that the end of my blue bettas tail is curling up, I tried to take the best photo I could but he does not stay still! He is in a 10 gallon divided tank with 0 ammonia, 0 nitrite and some 5 ppm nitrate. My other one has the same issue on the botton of his fin, but it doesnt curl up, it seems more stuck together but it doesn't look bad. The blue one is the one who is worrying me. I change their water weekly about 15%, so what could it be? The rest of their fins are absolutely gorgeous and unharmed.