It really depends on the species of the fishes, but I know some fishes grow lumps out of the blue, e.g. like my goldfish. The lump was a bit of a different color and quite noticable, and long story short- after I changed the food the goldfish changed to it's previous state within several days.

Turns out that some types of fish food can make them change appearance quite greatly and the salesman didn't bother to mention it to me.