The jury is still out on this. Some people experience issues, others don't.
The issue at hand is allelopathy, which is chemical warfare. If this is happening between BTAs, we don't know how it works -- so we don't know at what distance it becomes ineffective. In other words, placing your widow in a separate box in the same tank may not make a difference. Using carbon is a good
The only way to know for sure is to try it out. You'll see gradual affects, so keep a close eye on all of the nems. If you catch it in time, you'll be able separate the nems (into separate tanks) and the ones affected should be able to recover.