Honestly, there aren't many diseases we hobbyists come across. The common ones are fairly easy to treat. Ich is probably by far the most common.
You can chance it and see how long "it looks better for" before something major happens or nip it in the butt now.
For instance, my Achilles tang has ich. Like major ich. I feel bad each time I look at it in QT. Not only that, but now it's affected the desjardini AND two potters wrasses in qt with it. They all will be going thru TTM soon. Should be right now, but I can't so as of now they sit in an ich infested QT.
I know the Achilles will look better soon, but I know it's in waters with ich. So why convince myself it may not be ich and "he's looking fine"?
It seems a lot of people come on to ask about DZ, and expect to hear something else about what they have.
Also, without pics it's difficult to say what you have. Maybe you're lucky and your fish is truly the one "with some sand stuck to it".
Let us know how your fish do
Sent from your face