This is equivalent to the French formula "ne . . . que," which expresses the same concept of "only" when adverbially modifying a verb. "Il ne parle que anglais," meaning "He only speaks English." So, in Romanian, whenever you use "decât" in this fashion, it must be accompanied by "nu." It is the same thing when using "nimic" in this fashion in a sentence. It must be accompanied by "nu" as well.