Depends on some factors, but no they're not difficult.
I got a small head within a month of my first tank being cycled. At first it was in a 12 gallon tank with a 36w PC light and it did fine, though it didn't grow much at all in the 6 months it was in there. I was even spot feeding it once a week with a mix of zooplankton and phytoplankton. After it moved into the 55 I setup after buying a house it started growing great under the 260w PC fixture it was under and I had stopped spot feeding it and just pouring in a measured amount of the same mix I was spot feeding with before.
So the moral here is, better lighting will definitely impact it's growth, why spot feeding won't nearly as much. In a month or two I'll be putting it in my 180 under 4x 175 MH and 4x 160w VHO, so I can just imagine how well it'll do there.