The "levels" tag is already used for the exposure/curves adjustment tool common to photo-manipulation tool. Stack Exchange (reasonably enough) won't let me create a singular tag when the plural already exists. But here, I want it to mean something different. Please someone create the tag level and add it to this question.
Done. Out of curiosity, are you sure you couldn't actually create the tag? Even though it brings up a list of suggestions, I just hit escape and saved, and it created the tag. I'm not sure if that is actually a mod ability, or just how you have to do it in general.