ok, first a little background;

It's not unusal for me to come to turn on the fish light in the morning and have one, maybe two of my four large goldfish sleeping on the bottom. They always wake up when I turn on the lights. I did so this morning, as I always do, only to realize my prize Ranchu has a large hole in her tail fin, a few millimeters from the peduncle, and about as far from the top of the tail. It goes straight through both tails. A fin ray is visable bisecting the hole. He seems healthy otherwise, eating and such, and he put up one heck of a fight when I tried to take a closer look. Lil bugger got water everywhere. I don't see any white or red around the edges of the wound, but his tail is black so I'm not sure if I would see anything even if it was there. The hole is about 7 mm wide, and kind of an oblong shape.

Recently, I put my young clutch of mystery snails in my goldfish tank for holding. I've sold them already and I'm making the delivery in two days. There's close to 30 of them, but most of them are still small.

Could one of the snails done this to my fish in his sleep?

With my own experiences,and I've gone through thousands of snails,The only time that I've ever seen a snail do anything to a fish is if a fish is sick and not moving,or it's actually dead.A fish's first instinct is to swim off as soon as something approaches/touches their body,no matter if they're in sleeping position or not.Now you know to "never say never" because anything could be possible,but my instincts would look else where.
What size is your tank,any ornaments,etc,that could have possibly caused injury?

My tank's just shy of 50 gal, houses 4 fancy goldfish, 2 loaches and many juvenile mystery snails ( about 3 doz, but I'm selling 20 in a few days)

As far as ornaments, I've got some big fake palm trees but nothing with sharp edges. I've removed the plastic lotus flower I had in there. The petals are a touch pointy, but I doubt they could have done this.

Update on the fish: Leng Yu is making a speedy recovery after 2 treatments of Jungle Fungus Clear. The holes in his tail are almost fully closed up now. There's a bit of an indent where the new tissue has formed, but I still love him just as much

Well I guess the mystery will continue(I hate not knowing) , but I'm glad that all seems to be going in a good place for Leng Yu.
Keep up the good care.