A few months ago I got my first fish tank, cycled it, and introduced fish at staggered intervals until I had 6 neon tetras, 3 Corys, 4 danios and a dwarf gouramis. One of the danios just disappeared so I'm not sure what happened there. However in the last few days I've been noticing what I think is hole in the head on some of the tetras. It would be helpful if someone could identify if this is hole in the head, and if so what to treat it with. The ones that have it all have it on top of their heads between their eyes. Hopefully you can tell from pictures. Thank you in advance!