white scales

Hi, So this is my first time asking any questions on here, so if I placed this thread in the wrong location, sorry. Anyways, about a week and a half ago, I saw a female betta at the pet store (one of the big commercial ones), who was looking not so great. I purchased her because I wanted to...

So as the title suggests, i'd like to know what your opinions are on my tetra who seems to have a white spot of scales towards his back and a belly that protrudes weirdly. He's been like this for a couple of months now and im curious as to if there are any diseases that i should worry ahbout...