You're looking at type of hematoma (bruise), as everyone has mentioned this can be triggered by traumatic injury or stress. My guess is a little initial off and on aggression, and that little spike (the thing that gves tangs the name tangs....) on your Powder brown nailed the yellow a few times leading to a little internal blood leakage. that fact it has not gotten worse after the initial fluctuations in size probably indicates that there is no longer any aggression going on after they sorted things out, which is born out by your conscientious observations, and also lends doubt to the water quality possibility since one would expect it to progress and lead to other symptoms, loss of appetite etc. Nevertheless, Keep an eye on your water quality, it never hurts to do a partial change, that's the first thing I do in any situation just for my own peace of mind, makes me feel proactive. I think it will gradually diminish and clear up on it's own, keep us posted, good luck