Same for me. My fixture is 6in. above. Tanks only 16in. though. I have acro,s a good 17in. under the bulb however. A colony of green, brown and orange digita frags in the bottom corner, is about 24in. from the bulb and show good growth. Or at least good enough for me.
Derasa, hydro and milli,s on the bottom also look fine.
My fixture only has a couple of 18w actinic supplemental bulbs so while they give a nice evening look, dont add much to the par, so whatever light, is mostly from the Phoenix.