Hey Bobs I've actually seen this before. You're not having a water quality issue I gave a buddy of mine a duncan frag ( My Duncan had short fat "plump" tentacles like you described) After a few months with his duncan under T5's I noticed the tentacles became very long and thin in his tank. "ugly" if you will. The duncans in my tank were under 250w MHs at the surface loving it. Try moving them closer to the surface. I also read you're acclimating Radions? Great light btw I'm assuming this is a new fixture to the tank and when its operating at MH Comparable Par readings you will see an improvement! Hope this helps! And one more thing, ... "Any thoughts on heads receding and falling off?" If everything else in your tank is thriving this further proves (not a water issue) the Duncans are being starved of light.
Thanks for the tips I greatly appreciate the response. I moved some rock to the sump and placed the Duncan's near the top and I'll see how they respond.
P.s. I ordered a spectrapure 4 stage rodi with a live tds meter and inline pressure gauge. This should help me get my kh down to normal level as my tap water is high to begin with. And I'm on the hunt for a magnesium test kit in the mean time.
..And RODI ..It's kinda like the norm when dealing with coral. We spend a lot of money on these things you want to take good care of them. Everyone should use it for their reef tanks. Especially for top off (just have to add a buffer to it)