Good questions and comments...here are some responses:
I don't think the wound is getting bigger, but not sure. Not seeing any cottony fluff in it, just ragged skin and some pink fish flesh.
Water conditions have been at NH3=0, NO2=0, NO3=15-30 for a long time now. Tested today and they're at [0, 0, 5] (I guess b/c I've done wc's for the past 3 days).
Actually, I thought of something else that could be a factor. I had an ich outbreak a little while ago and treated with
API Super Ich Cure. Could that have caused damage? Didn't seem to bother any of my fish at the time..?
I've never seen the danios chasing or nipping the dg at all, so don't think that's an issue. In terms of whether to ISO him -- I've had some bad luck doing that, so kind of want to leave him in the 20g where he seems happy unless I get to a point where I know there's something in there causing harm, or I see a definite decrease in his health.
On the subject of Prime, my whole mini pet empire is built around API Tap Water Conditioner, so I'd really like to stay with that unless there's a clear indication that water conditions are causing my problems.
I'm posting some full tank shots below. Thanks to all who've taken the time to offer questions / suggestions. Please keep 'em coming!
With flash
Without flash (BTW, yes that's a pink dresser

)
Left side
Right side
