If you had ich, you'd have seen tiny white dots, sort of look like grains of salt, on fins or body. If one fish had ich, they would all end up with ich. From the pics I can't see any ich.
Fungal problems can happen, if there is a wound, or around the mouth area, but you'd have noticed fluffy whitish or greyish areas that would stick out from the fish body.
From the pic it appears the fish has clamped it's fins, which is a sign that it's feeling really unwell. May have been some internal parasite, may have been ammonia or nitrite spiking.
Do you test the water ? Do you know what the water parameters are ?
Harlequins are usually fairly hardy, being rasboras, but they may have brought some sort of infection with them. This is why quarantining new fish for at least six weeks is a very good idea. Sorry for the loss, I'd keep a close eye on the rest of them, and if you see any signs, if you take the fish out and put into a quarantine tank, might be able to save it. Post pics and describe symptoms and maybe there's something someone can suggest.
Of course if it happens very quickly, might not be time to do this.
Sad but true, we often never learn why some fish get sick and die.
