I was wondering what conditions cause a mono stay changes his color to a dark blackish, instead of the normal silver. Are they territorial fish, I've read conflicting info.
The main reason monos will turn from silver to dark gey or black is because of stress. It could be due to poor or inappropraite water, new surrounds, fright, illness , or a number of otherthings. Monos seem to be quite sensitive. As long as they don't stay dark for long periods of time, there is nothing to worry about.
Another big reason that Monos get stressed is if they do not have company. Monos can occur in huge schools in the wild and depend on each other in the wild and in captivity. If you don't have more than one it would be highly advisable to add more. Also are they in brackish water? Some Monos can tolerate fw for a little while but generally not for several months. Monos are only territorial if not kept in groups. As long as you have 4+ Monos they will pick on each other rather than any one else. Don't keep Monos with overly aggressive fish because this can stress them too!