October wasn't that long ago in "Acro time"... It has been metioned already, a couple of times, but many Acro's just kind of hang back for a while before suddenly and inexplicably taking off. Also, that green Acro you got from us is a very quick grower, plus if I recall correctly it had a few branches on it, an Acro with several branches will give the appearance of faster growth even at an identical measured growth, as it will grow in all directions. A 1/2" on a single "stick" frag doesn't look like much, but that same 1/2" applied to several branches pointing in all directions can have quite a different visual effect. Hang in there, keep an eye on things, and try to keep your levels as stable as possible. Minimizing fluctuations is half the battle. IMHO, consistency is key.
-Dave