Drain tile is a really misleading term! It’s not a flat tile; it’s a big ol’ clay pipe or tube, about 6" in diameter. The fish can hide inside it.
Assuming they realize that the shadow on the surface is a raccoon and not you with fish food. 
By the way, I’m giving away hardy pink :rolleyes: water lilies for the postage. I’ve had some luck with many leaves covering the water hiding fish from that damn hawk, but I doubt it would fool raccoons. Anyway, they need a home.
Heh! They even hide from me! (They’re free-range goldfish. I only feed them in the fall to fatten them up for the winter.)
Nor do I feed mine, although they seem to have grown so much I’ve been feeding when I think of it this Spring. They still hide from me, though, a good thing. 
I vote for large predator, such as heron, raccoon or neighborhood kid(s).
Alternate choice (less likely) - large hungry snake. Several years ago I gradually lost goldfish from a small above ground water feature and it was due to a snake (something I found out one evening when I was submerged to my elbows removing algae, and finally spooked the snake into lunging out of the water).
Fencing or netting might help prevent a reoccurrence.* Regarding herons, some people have used a large plastic heron stationed near the pond to discourage competition from the real thing, but I suspect these birds aren’t dumb enough to be fooled on a long term basis.
*or a Labrador retriever.
This happened to an acquaintance of mine. When they restocked it and watched it more closely, it turned out to be a combination of a heron and neighborhood kids fishing in it.
Too bad the kids didn’t chase the heron.