If the duncan fell off its plug, I would remount it with superglue. Also, if your getting such a difference between your test kit (160ppm) and your LFS 20ppm, I would buy a new, more accurate kit. I use Salifert for NO3 and it works pretty good.
I would not worry much about your Xenia, I purposely mow them down with my siphon hose each week, cut them with scissors, rip them out by hand etc.. they are immortal.
Since your tank is only 35g a couple 50% water changes every other day would be good to 'reset' your parameters and trace elements.