HI there! So I just got a white halfmoon betta yesterday. Unfortunately, my parents bought him on an impulse buy so they did not get a proper tank etc. I am currently sourcing equipment to get him the best tank. However, in the meantime he's in the little container and I'm starting to get worried. I've done a 50% water change today since its such a small container. But I noticed that he's got a black stain on the tip of his tail. Can anyone help me identify what it is? And what can I do to treat it?