Bob,
If you've never boosted your alk nor know what your alk level is then you can't say it doesn't work or has nothing to do with it. I shouldn't say its full proof.. but IME and in many others I have read boosting alk up a point and a half or so increases zoanthid growth. No, I don't know why.. I don't ask questions.. I just tried it out.
But to further prove my point. I have a frag of bullseye zoanthids. It has 3 polyps on it. I put my alk up from (hagen test kit) 110mg/L to 125mg/L and I now have 8 polyps on it... coinsendence? Maybe so... but all my other zoo frags seem to be "taking off" now as well. So therefore I say boosting alk increases zoo growth. I didn't just pull it out of no where... ask people on RC's zoanthid forum, they seem to agree
Can't hurt to try anyways
Scott.. where abouts do you have your frag placed that is not opening? Place it in a moderate flow/high light area unless it already is.