I agree , 10-15 ppm no3 is a respectable range. Corals need to eat! Good luck bud!I bet it isn't happy with the low nitrates/phosphates...alkalinity is where they usually like it. I had a Duncan that wouldn't open for months besides small stubby tentacles. Once I raised my alkalinity to around 9-9.5 and then the no3/po4 was around 5-10ppm and 0.05-0.10ppm, respectively, and now the Duncan is really happy and growing like crazy.
Duncans are good canaries...they pick up water parameter changes and let you know when they are not happy!
I've tried drilling holes in these small terrariums and constantly shatter or crack the thin acrylic. What's the secret?If you have room you could use a upside down clear critter cage with holes drilled in it, to allow flow, over the Duncan. Or any intank quarantine box that you can get over it. You could put a small piece of rock on top to keep it down. This would do a few things for you, not allow your fish to get near it, reduce flow, and also filter out some light. Duncans are pretty hardy coral.
This is what I use, with holes drilled in all sides.